You can get the train (1 stop) or the bus there, then it is a little walk through the shopping centre that is across the road (10-15 mins max), the walk is quite pleasant though.
I've not been myself but I've always enjoyed sea life centres!
We've been to the one there and for me personally, I have to say it's not really worth the money (it's fine if you have free tickets ). Can't remember how much we paid but we found it quite small and we were in and out of it pretty quickly. You can get your hand stamped for re-entry but we didn't bother going back. We only went to the shopping centre to have a look-see in the morning, didn't find it very interesting/much to see, popped into the Sealife centre and then decided to head straight back to the park.
I think the best Sealife we've been to is the one down in Weymouth.