Well, it wouldn’t get so acidic that we couldn’t go in the water ourselves, but it would make it harder for some creatures (like corrals) to survive. So it might mean that some or all corral reefs die out. This would be devestating for the local eco system, and for us, it would take away one of the most beautiful places to see, and make diving much less fun 🙁
I’ve just had a look at the projections for how much more acidic people think it might get, and the maximum change looks to be about -0.7 pH, (which wouldn’t happen for a few hundred years). This might sound small, but pH is a logarithmic scale (so a change of 1 is really times 10) and scientists have found out that corrals and other shell forming creatures are very sensitive to even small changes in pH.