I fish a few small lakes that have some old timers in them. I've had them pop-up near me and swim underneath me, since they are between 5 gallon pail and garbage can lid size, they get the adrenaline flowing. I've never had them act aggressively towards me, or heard of an unprovoked attack. I caught one last year that took a mulberry fly meant for carp. Now if you attached a rotting animal to your leg, all bets are off........I wouldn't put fish on a stringer attached to the tube.
On another note, the flotation device- you'll feel more comfortable as
it'll keep your head above water when it's trying to drag you down.
Seriously, you'll feel better. Tubes are a blast. I landed a 4' shark in one last year. I kicked to shore to land it. Couple of possibilities to make you feel safer-
Buy a tube with multiple chambers like a Fishcat 4 . If one fails, you'll still be afloat. I have a deluxe where the inflatable seat cushions alone will still keep me up. I've only had tiny leaks- never a problem. I've been in huge swells from ocean going tugs and even a cruise ship- you get use to being a big bobber. Oh, excuse me, indicator. Look into Force Fins- they are absolutely superior form of propulsion- huge difference over the cheap fins. Most tubers will gravitate towards them or something like them if they keep at it. Lastly, don't crowd shore bound spin fisherman , flying lead and trebles will get you before the snappers ever will.....