Lindsay,
If you are looking for a pair of boots to use with a float tube, might I suggest a pair of cheap walking boots. Good wading boots for the river have nice expensive felt soles or cleated rubber and these get trashed on the walk from the car to the loch in the Highlands. Walking boots also slide into the flippers easier. £30 - £40 pounds gets you a good pair.
Interesting. I use normal walking boots to get to the loch and then change into an oversized pair of cheap "deck shoes" I bought at Tesco. I found them much more comfortable than boots, and at a fiver, it does not matter if I leave them in the float tube back to go mouldy!
Failing that, I also have a pair of proper diving "fins" that are worn with stocking foot waders, no need for a pair of boots.