Very explicitly it is stated luck will give you more rolls
So effectively killing Calvus with luck 5 will, lootwise, effectively be like killing him three times meaning if a boss has a 1/7 it will roll that 1/7 three times. A sunken chance will be rolled three times meaning essentially every cast of a rod with an item chance of 1 will roll a 1/2000 three times
you only need the hat and cape for 100% water breathing and 10% swim speed.
from there with normal items you’d have 6 more jewel slots to work with for jewels that give actual stats and 5% swim speed each. (totaling at +40%)
I personally use 5 max waterbreathing and 7 max swimspeed musgravites (for now), full theurgist set, drowned or frozen magmamancer hat (probably frozem, unsure), powerful, virtuous and 3 armored
But this build is also supposed to be dark sea compatible.