Not much is square on a boat. I was fitting a floor timber, the piece of mahogany clamped to the frames. Each frame has a different angle and so does the keelson, which should be 90º, but it is close, but not ninety. The floor is clamped because it has to be bent into place, neither frame is square to the other. I have been using a nifty little shopmade tool. It is a tiny angle finder made from short pieces of worn hacksaw blade riveted together with a copper nail. To tighten it you just tap the copper. It is used to copy angles like a small version of a bevel gauge.
Another little tool was made from a recycled file by Carl. It is a tiny drawknife which I used to carve the form down enough so I could get a good scribe line on it. It is useful for inside corners where the low angle block plane won't fit.