Don’t forget ebony, which also does not float, or osage orange, which is possibly the toughest wood out there. A couple of centuries back, a good osage orange bow could be traded for a horse and blanket.

Here’s a chart, which black locust wood basically tops in strength.

http://www.woodbin.com/ref/wood/strength_table.htm

However, it should be noted that live oak has a higher strength-to-weight ratio that black locust.

I have read of lignum vitae which is supposed to be incredibly strong and tough. When dried and seasoned it will actually sink in water ! THere is a rough correlation between density and strength for wood. Teak is only about half as dense; it will float.

Rock maple, the stuff used for butcher block counters, is pretty tough stuff. Tried once to router some, and instantaneously smoked a high speed steel router bit. Swore a bit, then got carbide bits, which dulled to uselessness when I had done two feet !

Black locust is supposed to be very tough. Also Ironwood. Both North American.