Shipwrecked in Saltmarsh

The massive, teleporting spider doesn't reappear. The party remains in the web-filled upper cabin, surrounded by thick webs, old furniture, and the bodies of their earlier foes.

     Fianna takes a moment to blink Fi'Finella out of existence from where she is and then back into existence with her.  "We need another pair of eyes with us here," she tells the imp familiar and has her climb into her backpack, head poking out and keeping an eye behind Fianna...

     "Well guess we have to be even more careful now," Fianna says to the others as she readies to move back down into the ship.  "Staying here won't find us what we need..."

Kelnan

(Do I have any idea what the teleporting spider is all about? Might have read something about magical wildlife somewhere.)

"No more wasting time then. We do the job and leave before these things overtake us." He scans the room for a crate that might contain the missing box, summoning a Mage Hand to help push webs out of the way without getting tangled up himself.

(Kelnan may make a Nature or Arcana check regarding teleporting spiders.)

He clears much of the room of the webs with his mage hand, without incident. The room contains a weathered table and four rotten chairs, as well as piles of trash and debris. The table is still covered with charts and maps, stained and covered with dirt and mold. There is nothing here that resembles the description of the box you are looking for.

Fi'Finella keep an extra pair of eyes out for threats. She reports nothing unusual in her recent surveillance of the ship from the air. The Dwarves are maintaining their distance in the skiff, and further out, their vessel. They wait patiently for your return.

Arcana: 24

Kelnan has read of creatures that possess the magical ability to phase in and out of the Ethereal Plane. These sorts of creatures seem to appear out of nowhere and quickly vanish after attacking. Its movement on the Ethereal Plane before coming back to the Material Plane makes it seem like it can teleport. During combat, they can magically shift from the Material Plane to the Ethereal Plane, or vice versa. He recalls the name "Phase Spider" for one such creature.

The cabin, now clear of webs, remains quiet. The spider does not return.

“The only thing to do is soldier on. Try to stay behind me this time, Dain. I know you’re sort of weak.” Molo holds a protective hand back towards Dain as he proceeds to lead them further into the ship, past where they fought the spiders before.

     Fianna follows Molo and (presumably) Dain…  eyes sweeping around, wary of threats both phased and non…

Kelnan shares his knowledge of the phase spider, warning the group to keep eyes all around and backs to the walls. He also offers to sweep ahead of the group with his Mage Hand to clear webs, recasting it as needed.

The troop descends the stairs again, with Molo limping in the lead. Kelnan begins to clear the cobwebs with his Mage Hand until the wounded hairy spider (the one the size of a large dog that attacked you earlier) springs out of hiding in the far corner of the cabin to attack the Mage Hand. (It fails.) It pushes open the cabin door on the starboard side aft (lower left in map) and vanishes into the gloom.

“After it!” Molo readies blade ward and moves through the door after it

Kelnan slaps Molo with the Mage Hand.

"We're here for a box, not a bug hunt. No need to agitate the wildlife or give that phase spider an easy meal by running off alone."

     "Searching will be easier if we're not constantly attacked by hit and run tactics..." Fianna will respond, moving forward to edge into the room the spider fled into.  

And that's when Molo, caught up in the argument, falls through the rotten trapdoor into the flooded hold below. 

Make a DC15 Acrobatics check to catch yourself - at disadvantage for being caught up in an argument and being slapped around with a mage hand...

Molo acrobatics (at disadvantage) 8 

 ”Don’t slap me! I’m-“ 

Whoosh, down to the bottom. 

Molo's darkvision kicks in, and he finds himself treading water in the cargo hold. This deck runs the length of the ship and features no bulkheads or walls. Dark, murky seawater fills it to a depth of three feet - just over Molo's height -  sloshing against old, moldering crates piled against the ship’s port hull, where the water is somewhat deeper.

His comrades look down upon him from the deck above, where the rotten cargo hatch gave way.

Molo continues to tread water in the dark hold, while the rest of the party looks down at him through the broken trap door. Molo cannot get back up without help (although he can make out a ladder near the stern.

Some daylight filters through from above - the rusty metal grate on the top deck is situated right above the trap door - this is clearly where cargo was once loaded and unloaded.

The cargo hold is web-free. Both locations currently appear to be spider-free as well.

Molo’s pretty comfortable in the dark and attempts to look around in the water for anything of interest efore heading for the ladder. 
perception 19 

"Hang on laddie," Dain says down to Molo, seeing him ready to move off in the lower deck.  "I've a rope here.  Why doesn't ya just climb back up here and we can carry on as a group?"  Dain will take his rope out of his backpack and drop a line down to the kobold so that the offer is there if the little one chooses to take it.