Swim near them so that they orient on you, then make sure you have an empty inventory space selected and hold down E--you'll hold your hand out to them and they swim close so you can pet them :)

Oh! I also found out that you can go back and see what he said by pressing tab, and going into the discussion tab!

It's Left Trigger for controllers.
When you are not selecting an item in the inventory, hold the LT until the loddle get close and let you pet it.

Thanks I wasn't sure how to pet them. I did try the empty slot and 3. I guess I didn't wait long enough.

Thanks! Needed this game after Resident Evil :D