Yeah I learned all the water-based Shark techniques before they were mad exclusive to Kisame and the SC xD
Well, the only character shown to posses the Shark Contract was Kisame, Shark summons were used by him tipically to deliver messages and notes to his allies, but were shown to also be very aggressive when required. Showing that they were even capable of devouring their own master, this makes them a deadly contract, even for use the users. It was shown that Sharks can be mixed into Water techniques, such as using the Wall of water sharks to hide a real shark, which later escapes or attacks unseen. Sharks are water based animals of course, so naturally there must be a water source for them to be summoned and made use of. They have been shown to be quite large, even larger than Kisame. Given we didn't see a lot of them, we can guess there are even larger and stronger Shark summons in the narutoverse.