There’s a possibility that I’ll have a chance to play in a Saltmarsh campaign, and the potential DM asked me what kind of character I’d like to play.   Given the nautical nature of the adventure, a Swashbuckler rogue sounded interesting to me.  I like to be able to play characters have some casting ability, so in thinking about how to accomplish that with this character, I’m tentatively planning to multi class into a bard who would take college of swords. I am more interested in the concept than in optimization, but I’d be interested in recommendations about when to take what levels.  My current plan is to take 3 levels of rogue to get to swashbuckler, then 3 levels of bards to get to college of swords, but then I’m not sure what to prioritize, although some of that may be story driven—if we even get that far with the story! 

Any suggestions or ideas?  If it matters, I plan to choose Air Genasi for race.  Potential feats are welcome as suggestions as well, although I may want to prioritize ASIs.

It’s a fine build. Nothing wrong with how you want to do it. Could be lots of fun. 
Extra Attack on Bard level 6 icreases your damage output, so does the ASI on level 4 of the Rogue.
So if you want to start rogue you could try this:

• Levels 1-4 Rogue
• Levels 5-10 Bard
• Rest: Whatever you want and depends on what level your party will get to.

Two-weapon fighting could be a fun way to go with this multi class. Give you 3 attacks at level 10 which isn’t bad + sneak attack. Make Dex and Cha your two highest stats with Con 3rd highest and you’re golden.

The downside of two weapon fighting with this combo will be you have A TON of uses for your bonus action outside of attacking with it.  Bardic inspiration, (not used for a flourish) some spells like healing word, and the rogue cunning action.  It is both a positive and negative thing to having so many options but that all comes down to personal playstyle.

I'd say take your 3 levels of rogue, then 3 levels of bard... then just see which skill set you find yourself relying on more. If you find yourself as your party's best melee DPS, prioritize Rogue going forward and focus on maximizing damage output with sneak attack. I do agree that it would be worth putting 3 more levels into Bard eventually, since it would get you Extra Attack, and the more chances you get to land a sneak attack, the better. However, if you find yourself regularly burning through your spells and wanting more battlefield control, prioritize bard and treat the Rogue levels you've taken as an additional way to sneak in some more damage. In that case I'd say it's still worth it to eventually put two more levels into Rogue, both to bump your sneak attack to 3d6 and to get access to Uncanny Dodge.

I generally don't like the feel of multiclassing half-and-half with classes. I generally find it works better to prioritize one over the other, and to prioritize the early-level abilities in your secondary class that synergize best with the playstyle you're going for. But it's also good to not get married to a specific build, since your goals might change depending on the game you're playing and the composition of your party.

If you only play until level 10, I think either Rogue 3/Bard 7 or Rogue 4/Bard 6 work well. As far as advancement goes, I would probably take 3 levels in Rogue first, and then 3 levels in Bard. I think holding off on the ASI is worth getting into spells, jack of all trades, inspiration, and flourishes early. 

If you go higher than 10, you will have to think about whether you want to focus on spellcasting or melee combat. The more bard levels you take, the worse you will be in combat. You need to keep advancing sneak attack to be relevant. And why even go in melee when you can cast Animate Objects? 

If you want to be a primary spellcaster with some great battlefield defense and maneuverability, I would opt for maybe Rogue 4/Bard 16. 

If you want to be something of a gish, I think Rogue 13/Bard 7 would work well. 

I haven't played Saltmarsh, but supposedly it's a 1-12 affair, so plenty of room to sketch out optimization up to T3.

What I'm hearing is, swashbuckling with spells, not necessarily a build with the specific class features of those (sub)classes. When I hear swashbuckling, I think one-handed weapons with an open hand for rope swinging etc. I think mobility. And I think ability to flummox opponents creatively, rather than just stab them to death.

Where I'm going with this is, I think that Fighter (Battlemaster) is a much better second class to bolt onto your Bard than the Rogue is. Combat Maneuvers can do so much to spice up regular attacks... you aren't stabbing him, you're distracting him with a flurry of blows while your teammate scrambles away! You don't just slash at him, you slash the top of his hand causing him to drop his weapon as he hisses with pain! Combat Maneuvers take normal attacks, and punch them up with a level of (very flexible) cinematic storytelling that just goes perfectly with a swashbuckling character. Being a Rogue is great, but honestly I feel like more skill  proficiencies and Expertise are a big reason why I like them, and that kind of doesn't socket well with the Bard's Jack of all Trades which rewards you for NOT having proficieincies. Cunning Action is cool... but Swashbuckler already has some mobility options, and needing to Bonus Disengage or Dash is a lot more situational than it looks on paper. 

Swashbuckler: Blade Flourish uses are tied to Charisma modifier, not to class level, so it's a fairly multiclass-friendly feature. The regular Bard 5 Font of Inspiration (Inspiratin becomes a short rest refresh instead of long rest) is vital to using the subclass feature, so you need at least 5 Bard levels. 1 more gives you Extra Attack, which could let you leave Fighter sub-5, so that's worth going to 6 over... but 7th and 8th level are only worth a 4th level spell slot a piece, which may or may not be worth it based on who else you have for casters in the group. It certainly would be funny to use Compulsion to make an entire enemy crew jump off their ship during a boarding... but if you really wanted to, you could skip Bard 7 and 8, and squeeze in a couple levels of Rogue (meh) or Hexblade (meh) or Storm Sorcerer (meh?).

Recommendation:

• Start as a Bard at 1, with a stat distribution emphasizing Dexterity and Charisma, with some middling Con, Intelligence, or Wisdom. A Mark of the Storm Half-Elf would add some fun nautical spells to your class list that a Bard doesn't normally get (Fog Cloud, Gust of Wind, Levitate, Sleet Storm, Wind Wall, Control Water, Conjure Minor Elementals), in addition to on-point stats and some other fun nautical abilities. Otherwise, Lightfoot hafling plays well with Swashbuckling shennanigans by letting you move through enemies, or plain old Human never goes wrong (maybe with Prodigy?). A Ravnica Background like Simic Scientist would add a lot of great swashbuckly- or nautical spells to your list as well, including Expeditious Retreat, Alter Self, and Water Breathing.
• Real fork in the road here... Bard 2 makes sense to get Jack of All Trades quickly, but it starts to lead you down the path of getting to Bard 3 ASAP, which would then agitate to pushing forward to Bard 4 for a Feat, Bard 5 for Short Rest Flourishes, and Bard 6 for Extra Attack. Just no reason to slow any of that down for Fighter!
  • Or, Fighter right at level 2, and stick with it to Fighter 3. Combat Maneuvers will feel far flashier than your Swashbuckler flourishes, and start off as Short Rest resources right from day 1, so being "a swashbuckler" by pushing Fighter 3 makes a lot of sense. Pick up Defense Fighting Style along the way (Swashbuckler will give you Duelist), and enjoy Action Surge as well for some real cinematic turn potential. But don't take it past 3 until you know you're going to hit 12th level and don't want a third class, Bard 8/Fighter 3 gives you more than Bard 7/Fighter 4 will at the end of the day
  • Or, Hexblade at level 2? Multiclassing a third class would seem like it would slow your progression, but honestly, being able to swordfight with Charisma will ACCELERATE your progression in the long run, by letting you just max your Charisma instead of leaving it at a +3 for ages while you focus on Dexterity. You can't really fit more than 2 levels of Warlock, if you want invocations, but Mask of Many Faces and Misty Visions could both be a lot of fun on a magical swashbuckler?

At the end of the day, if I were drawing a level 12 character on paper at the end of Saltmarsh, it would probably be a Mark of the Storm Half-Elf Bard (Swords) 8/Fighter (Battlemaster) 3/Warlock (Hexblade) 1, landing at 9/14/12/12/12/20, wearing Half Plate (or a Breastplate, if you want higher AC and don't mind Disadvantage/poor thematics) for AC 17, 78 HP, and wielding a charisma-based rapier for 1d8+5+2+(1d8 Swashbuckling)+(1d8 Combat Maneuvers)+(4 Hexblade's Curse)+(1d6 Hex if necessary). Elven Accuracy for the +1 Cha. I think I'd start as a Bard at 1, Dip Hexblade 1 at level 2, then Bard to 4, then Fighter to 3, then Bard to 8. Not the most elegant progression path, but I don't think you're left feeling like the build "hasn't come together yet" at any point, you're clearly a Charismatic magical sailor duelist at all points in your career. 

Played a lvl 8 one shot with a variant human with three levels of rogue for swashbuckler and five levels of swords bard. Took defensive duelist as the first level feat and got studded leather and a rapier. 

It was a pretty nasty build. I could cast bane or blindness at the start and kite opponents with the extra speed and pushing attack. Defensive duelist gave them an effective AC of 19 not counting defense flourish. And I even though I only got one attack most of the time that attack was doing 2d8+2d6+6 damage. They also had expertise in four skills which is pretty silly.