Everything!
I only once encountered a sea lion underwater and it was memorable. I was doing a drift dive in Dodds Narrows near Nanimo, Vancouver Island and suddenly a large sea lion swirled in front of me, seemingly nonchalantly, checked me over and swam off. I pondered the apparent lack of effort by the sea lion as I was being flushed through the straight by a irresistible current. Powerful!
I also remember letting go and allowing myself to be swept out into the blue with the other divers at Elfin Stone, Egypt. Out in the blue a quartet of dolphins were in pursuit of tunas. Underneath them all were grey reef sharks.
But I have on a few occasions found myself side-by-side with whalesharks. That sure gets the adrenaline pumping.
I much enjoy the smaller creatures too, like nudibranchs, sea horses, sea moths, etc. Spotting camoflaged creatures like frogfish, stonefish, etc. is also exciting.
Now I have a wish list of creatures I wish to encounter underwater. I think I'll make that a new thread.