Ph levels have been lower in past, therefore these sea lions you speak about must be figments of your imagination. They went extinct a long time ago.Sea level rises aren't what worry me the most.
If the Arctic melts sea levels will DROP. If the water on the land of the Antarctic melt, the then sea will rise.
So, the reality is, if the Antarctic and Arctic melt a little, the effects might not be noticed at first. Later on when more and more land based ice is melting, then that is when problems will occur.
The problem is the PH levels of the oceans. I read about sea lions dying off because they can't cope with the level of change of the PH in the oceans. Other sea creatures will be the same, and this is the biggest and most worrying part of polluting the atmosphere we're going to see first.