Old Boats, Leaking Hulls, and Drain Plugs

New to Hobie Forum and just learned that my (very?) old Hobie 14 we bought last year is leaking. We have it in a slip on a lake which I just learned is ill-advised. Early in the season, the hulls had taken on water. I emptied them and used plumbers teflon tape on the hull plugs which seemed to work just fine - no apparent water for probably 6 weeks. We just took 4 adults out and hulls became submerged while sailing. When docking the boat, clearly the hulls had taken on water, a lot, after having sailed for less than an hour. I am guessing the water entered some place other than the drain plugs tho not certain (when docked, the drain plugs sit just proud of the water line). Guess we should beach the boat when not sailing. Any suggestions for order of checking the rest of hull for leaks? thanks, d