Stall, Speed, and Scaling
To support your main engine's win condition, you're going to want to look for ways to disrupt your enemies with CC (with Freeze or Slow), help speed up your casting or further accelerate your scaling.
There are a lot of options that are interchangeable for these goals, but here are some examples that Rarran used:
Clamera (Slows + Aquatic Triggers)
Bullet Time (Slows on Harpoon cast)
Barnacle Crusted (Speeds up Aquatic casts)
Keen Eye (Grants your Shields and Heals Crit potential)
Gumballs
Gumballs can be an attractive choice in defensive builds because they can scale your win condition with more Max Health, Crit%, and Shields. You will whiff on some Red Gumballs though since you don't have any Weapons.
Enchantments
Throughout his run, Rarran was able to find Turbo Shipwreck, Shielded Seaweed, and Radiant Sea Shell.
It's hard to control when you can get Enchantments (outside of level 16 and first death) and which ones you get, but it's important to note that these can have huge impacts on your success.
When in doubt, always look to Enchant your most impactful items that have the most scaling potential.