The problem is that waterproof plasters need to be made of a non-elastic material (otherwise they wouldn't be waterproof) but, as we move our bodies, our skin is constantly stretching and contracting.
I won't buy waterproof plasters; I only use the 'stretch fabric' ones. If I need to cover a wound with a waterproof dressing, I'll use a Melolin dressing (shiny side down, of course) and hold it on with stretch fabric tape.
savlon do a waterproof plaster they are called Advanced plasters for faster healing and they really work. My husband put one on his toe two days ago, has had two showers in the last couple of days and the plaster is still on. These plasters are also waterproof and breathable and helps stop further bleeding.
It can be a right pain. I cut my finger on a sharp knife whilst washing up this morning, and to finish washing up meant I had to put about 3 plasters on, and keep stopping to change the plasters! The only truely waterproof ones I have ever had I bought in the States. They have so many good things over there that we just can't buy here.
None of them work IMO, even ordinary plasters don't stay put even if kept dry. When I was a child those stretchy elastoplast things never came off even when you wanted them to and left that yukky sticky gunk on your skin for days.