How do I stop myself from being slowed down in the Sea?

Solution 1:

Adventuring underwater takes longer, so each adventure you go on costs 2 turns.

This can be temporarily removed with the fishy buff so that underwater adventuring only takes the normal single turn. With that said, unless you want to spend a lot of real time and meat tracking down sources of fishy (most of which have per day limits), you are better off just dealing with the turn loss.

Underwater adventuring has some other effects in addition to the double turns, including lower item and meat drops because of pressure penalties. These penalties are reduced by using equipment that "makes you a better diver". There are also specific gear and buffs that drastically improve underwater meat and item drops to counter the penalties as well. Most of that gear can be found in underwater zones, but you can also pick it up for meat at the mall.