You could model the "being dragged/pushed into the water" thing as the taken out result of a previous conflict. Most things will be (pardon the pun) dead in the water, if they are faced with a pack of hexensharks, so the result of the actual physical conflict is pretty much determined by the hexensharks anyway. If the opponent in question is still considerable in the water, then you can work out the actual conflict, of course.
As others suggested, this will probably have to be a social conflict, using the siren tactic or whatever. You could also have part of the pack on land in human form to tackle them into the water.
Though I would center on more maritime adventures generally. Otherwise it kind of becomes a "Knight Boat" phenomenon. If a foe can escape on land, they have the advantage over your guys. So their incentive should be to catch them before that. Give the opposing force an island base, maybe even an underwater thing. Or an oil rig platform. Or simply a ship. That way, they have their territory to protect or reign over, and whatever happens on land is someone else's business, or they have to get help from land-based forces.