Really beautiful stuff here honestly! My favorite tracks have to be Dear Mr Tuna and Join my Crew. The chord progression is so catchy in Dear Mr Tuna and all the instruments work so well together; really takes me somewhere tropical. Join my Crew has this pirate swing feel to it that I just love. It's fun and a very fresh change in this OST. There's so much variety and lovely textures found throughout this entire soundtrack so well done!
Only real negative thing I can point out is there's some songs that have mixing issues such as low rumble that muddies the mix. I'd also suggest adding volume automation/velocity control to your instruments to add variations in dynamics. I don't know how easy mixing frequencies is with MuseScore but I'm sure there's an efficient method out there! (Or cave and become a DAW user, hahaha)
Again, nicely done. I can tell you put a lot of effort into telling a story with your music. I think this OST would work brilliantly in an ocean themed game!