Learn Turkish: Vanalar kapalı olduğu sürece gövdeye su girmez. - As long as the valves are closed, water does not enter the hull.

You use the helper verb olmak (to be/become). Since kapalı is an adjective, you add the ending to olmak, creating kapalı olduğu sürece (literally 'as long as its being closed').

Since vanalar (valves) is plural, shouldn't it be oldukları sürece instead of just olduğu sürece?

In Turkish, when the subject is non-human and already pluralized (vanalar), the verb or verbal noun usually takes the singular third-person form (olduğu). Using oldukları wouldn't be strictly wrong, but the singular is much more natural here.

Why does gövde (hull) have the dative -y-e ending here?

The verb girmek (to enter) always takes the dative case for the destination you are entering. In Turkish logic, water doesn't 'enter the hull'; it 'enters to the hull' (gövdeye girmez).

Why use the aorist negative (girmez) instead of the present continuous (girmiyor)?

The aorist tense is the go-to choice for general facts, rules, and physical truths. Because this sentence describes a general mechanical fact, girmez is much better than the continuous girmiyor (which would sound more like 'isn't entering right at this moment').