Medic-nin Ranma? Yeah, that is cool! Very cool! And somehow Ranma seeking out and getting the tutoring of a sannin in order to fight Orochimaru fits, and Tsunade seems the best one for him, since he isn't the sort to tolerate Jiraiya due to the girl-curse and Jiraiya being a 'super pervert'. (He learned more from Cologne than Happosai) But you should know I thought that it would actually be harder and require _more_ strength to move the boulder underwater, and the girl form is indicated to have lower physical strength.
Ranma and Jiraiya are a combination with great potential for hilarity, but Jiraiya is elsewhere with Naruto and has little reason to show any interest in Ranma. What brings Ranma to Tsunade's attention wouldn't really attract Jiraiya. Mind you, saying that, Ranma wouldn't exactly have a difficult time attracting Jiraiya if he felt like it, but he doesn't and won't.
As for the underwater bit, I admit: I haven't studied physics, any physics, in nearly a decade now and I wasn't exactly at a high level then (it was in fact a thoroughly mediocre level). I may be completely wrong but either way doing something underwater is quite different to doing it normally. It's not an even comparison, and we've never seen Tsunade doing anything that strained her physically. Even using Gamabunta's sword to take down Manda didn't seem to be something that really stretched her.
The reason I'd kind of like to see the neko-ken dealt with is I was hoping Ranma would wind up mastering it and then deciding to supplement it by getting the cat contract from Hitomi in some sort of deal with her, perhaps where she is requesting some really cool technique or forbidden style like the Umi-Sen-Ken in order to surpass the main house, and exchanges the chance to sign the cat contract for it, because the idea of S-class Ranma with cat summoning and mastered neko-ken appeals to me even more than the idea of a medic-Ranma, and both combined would be ultimate coolness. Smile But then again, it is your fic, and your writing is good enough I don't care if you take the suggestion or not.
Unfortunately, no. Ranma won't be summoning cats. I have plans for Ranma and summons - not cats, but I think what he does get is amusing - but they won't come in for a while yet, I think. But I think the humour potential in my idea is high (I would, wouldn't I?) and it'll get there sooner or later so you can see for yourself.