Is there any list out there that tell you what useful things you can get from Sea Coins?
I already got all the collectibles you can buy from them, aswell as the omnium and astray item.
Is there anything else I should be getting aswell, or can I just trade them for pirate coins?
Thanks in advance! 
Mats?
Theres alot of things You can buy off of a Ship Vessels at almost each dock worth like 8k sun coins for a omnium star at punika, or tear of abyss also for 8k at the same spot used to trade for relic tier crew. Theres also collectibles like masterpiece and giants heart worth over 3k giennach coins each at the island with Pest Control daily, cant remember name. Theres much much more to list but i have no source for that right now.