Mounts jumping use the lesser of their own Acrobatics or their rider's Ride modifier. So you probably won't get the full benefit out of the Wand of Jump on the shark.
There's no reason sahuagin spellcasters can't cast fly themselves, but I like the coolness effect of them soaring out of the sea and jumping on board.
You can have pairs of them racing along and one give the other a push -- aid another checks are great.
Sahuagin bards can inspire competence instead of inspiring courage, then switch once enough of their guys are on board.
Sahuagin could use lassos to try and grab someone on board and drag them into the sea.
Mass bull's strength on a bunch of the jumpers could help too.
A storm rages in the sea. Lightning cracks and the boat rocks in crests and troths that are half as tall as the ship. The sailors shout orders and tie down the hatches. A few of them attempt to throw the anchor overboard as a wave crashes broadside to the boat. A foamy spray is cast across the deck. As the water settles, the closest give shouts of alarm "Sahuagin!". A flash of lightning reveals 6 of the fishmen spreading out from the main deck. <roll initiative>
A storm? Sharknado!!
Now seriously, Just Do It. It sounds so cool (big sharks jumping aboard the ship carring sahuagins...) that now one will wonder about the mechanics involved.
Just a question, how will you manage the onboard sharks? for movement I mean.
Most any kind of sailing ship will rock with the waves and the bow/stern/sides will all come near the water line level. Just have a moderate swell and have the Sahuagin time their jump into the ship with the swell.
My mental picture for this scene is something out of an extraordinarily cheesy, Sailor-Moon style anime, whereupon jumping there's a lens flare, some primary color background change, and then they all land in various poses on the ship.
Kinda like a Sahuagin version of the Ginyu Force from DBZ.
Buffing their CMD is a much easier means to do that than all you're going through if all you have in mind is preventing them being tossed overboard. All boosting jump will do is let them hop back up the next couple of rounds.
Also...it's not like it's a tactic that will stop them for long. Even without the ability to jump back, it will at best take a few out of the fight while the players focus on those that remain. The ones that fell off would climb back on in the next couple of rounds, anyway.
So why discourage them from using a viable tactic that will only be a temporary measure anyway?
I'm assuming this is Skull and Shackles? I'm a player in that right now, and we've dealt with Sahuagins boarding the ship and generally harassing us for a while now. Grindylow are somewhat annoying, but man, Sahuagins are just plain dicks. Maybe I have a bias because we were able to convince a handful of grindylow to join our crew after we killed their matron, but the sahuagins just aren't amenable to either diplomacy or intimidation, the two methods we've used to gather most of our crew.
Anyway, despite the grindylow we faced in the first book using the bullrushing/trip-and-drag tactic against us earlier, we've never thought to do that same technique to the monsters that attacked our ship. We usually just straight-up kill them. Much simpler that way.
We had a Sahuagin recently hail our boats and just ask to come aboard. Once aboard he looked around and stated" Nice ship you got, be a shame if something happen to it . BUt dont worry me and my boys keep the riff raff out of this part of tne Ocean, we consider it a public service."
He then offered his service as a guide through the ocean so our ship did not any reef or run into any problems.
why climb the boat?
should just smash bottom and meet the draylanders in the wet world.
(then again if this is a 5th level party. might be an overkill)
Why makes rolls? Just say it happens and evoke the 'Rule of Cool' with it.