Pressurizing isn't so simple.
Pressuring is actually quite easy. Use a piece of rubber hose about 6" long and small enough to fit in the drain plug. Wrap the end with duct tape or electrical tape so that the hose fits snugly into the drain opening (you can actually thread it in). Put your mouth on the hose and blow in with a dozen or so good breaths. If you have a second person, put your finger over the hose while the other person goes around the boat with a spray bottle of soapy water spraying for leaks. Top off the pressure as necessary. If you're working by yourself, use a pair of vice grips to pinch off the hose and then go around and spray the boat. I have done this several times and it is very simple.
It is much better to actually find the source of the leak than to go around randomly squirting caulk into various places.