This is what people ask me in the pool all the time. Water resistance is different than the one that gravity provides but strengthening muscles in the pool or in the gym is basically the same. The only difference is that your mineral bone density doesn't improve in water like it does in the gym, which could make you more vulnerable to bone problems in the long run.
There are other ways to boost your muscles in the pool like by using resistance equipment such as aqua dumbbells and hydrotone. See more