From my boat repair experiance when using marine sealent you cann't allow the object(s) to move as it drys, and there is no such thing as too much. Any brand name stuff should do, I think (mabey) 3M makes one, that is good.
If you're tubing is leaking without clamps than they won't be much help in preventing leaks. If the tubing is leaking around the barb than I'm assuming it doesn't form a perfect seal. If you've used this tubing before it may have stretched out a bit so cut the ends off and the "new" ends should form a tighter grip and better seal. If this isn't the case I have no clue