I will use Frostbite as an opportunity attack:

If they get hit it will do {2} damage...harpy has to do a constitution saving throw. 

I will hit with a light hammer:

To hit: 17 | Damage: 6

I will slash with my Rapier as a bonus action:

Attack: 21 | Damage: 9

And (as movement?) I will pull my Javelin of lightning out of the harpy

Harpy's too far away for pulling the javelin out or the rapier. You throw your hammer at the harpy. It hits one of the harpy's wings, and the harpy screeches and plummets into the ocean- with your javelin still inside. Luckily, the hammer flies towards the survivor and they catch it.

Also, I started another campaign if you want to join. Link: https://www.dndbeyond.com/forums/d-d-beyond-general/play-by-post/192993-lord-of-nessus-a-d-d-5e-homebrew-campaign

I pull the harpy up on the ship and weigh down the rope that the harpy is connected to, using a bedroll with rocks in it.

And I pull out the javelin.

I ready myself, so if the harpy attacks me, I use the longsword (+1) and attack back.

You pull up the harpy and weigh the rope down and pull out the javelin. The water on the harpy suddenly crackles, and there's a pop. You have a cooked harpy. Captain Rosa comes out from behind a barrel and laughs. "Well, now that's something." Finally, the survivor reaches the ship. 

For killing the harpy, you gain 200 XP (that means a level-up for you). 

If you want to join another campaign, join this link:  https://www.dndbeyond.com/forums/d-d-beyond-general/play-by-post/192993-lord-of-nessus-a-d-d-5e-homebrew-campaign.

"Hey, who are you? Ah, and can I get my hammer back?"

(OOC (out of chat): I'm going to be bot-ing for character)

"My name I can't disclose, but here's your hammer." And he hands you your hammer. 

I take my hammer

"Ah, understandable. I should introduce myself My name is Contritus." I point to the Captain, "This is Captain Rose."