I agree with Naji. Don't engage a seal or sea lion. Although they can be playful we don't speak their language and we don't know when we are signaling aggression. They can deliver a nasty bite. I have never heard of one mauling a person though.
Dolphins can be the same way. The spinner dolphins that we have here will flee when bothered but that is not the case with bottlenose.
For sharks the advice I've been given and have followed is to take a calm assertive stance (like the dog whisperer). So far that has worked for me but I haven't seen an aggressive shark just reef ones.