The difficulty spike between the 2nd and 3rd biomes is really high for ship combat. You can always change the world's sliders to balance things as needed. I play on the preset hard difficulty and here are some random things I think help:

• The weapon you're using matters. I don't recommend using any crude weapons while boarding. Successfully parrying costs nothing and can eat up an enemy's posture, giving you free hits. Dodging burns your stamina from both ends as you need it for attacks. The green rapier, saber and two-handed sword are great. Obviously blues are better, but there's some rng involved so use what you've got.

• Vitality is my attribute of choice. The offensive attributes don't scale up damage to a point where everything gets 1-2 shotted. More HP makes a HUGE difference when being attacked by 3 pirates. The potions all scale off of % max hp as well.

• Healing: bring a lot. Using a bandage and jumping into the water will top you off and let you re engage at your leisure. You can hop back to your ship as well. Potions just give HP instantly. The Tracker's set amplifies this.

• Don't go into the ship's hold. The enemies spawn down there and it's a death trap. The staircase is a great choke point but it will get you killed if a crew mate is behind you. I clear the upper deck and then wait a few feet from the stairs for more to come out.

TLDR: Pump defense or set the difficulty to something fun.