Yes, I WILL need larger mirrors for driving. But there's a big difference
between seeing what 50 ft behind you and seeing a boat that's narrower
than you are, 10 ft back.

One of the useless skills I happened to learn in life is how to back a semi
trailer through a slalom course without knocking over any cones. :-)

If you're backing caddy whumpus, the trailer will show up in one mirror or the
other before the angle becomes too extreme. You can then correct accordingly,
(turn the wheel into the mirror with "too much" trailer showing).

If you feel as though you must keep track of both rear corners of the trailer
when backing up in a straight line (recommended), there is a little trick you
can use that won't involve bizarro mirrors.

Saw off a couple of lengths of old wooden broomstick. Hang a red plastic
pennant on each one. When you get to the ramp or some other backing situation,
insert these
broomsticks into the left and right sides of your trailer. They should stick
out far enough that you can see the flags in your mirror. As soon as you start
to turn, of course, one of the flags will disappear but they will provide a
useful reference to let you know when you are backing in a staright line. You
many have to rig up a couple of fittings on the trailer to accommodate the
broomsticks.