_ wrote:

I have a cobbled-together setup that almost works. I coach swimming, and I
have a small video gadget

Well that's the first problem. Those cheap video transmitters are
barely worth what you paid for them. There is a huge quality and price
difference between models. Pay $50 and you get $50 worth of video quality.

The ceiling and the upper half of the inside walls of the pool are panelled
with strips of aluminium sheathing - about 6 inches wide and 20 or more
feet long.

Ok, so not really a perfect environment for wireless.

What's the best guess for improving the picture?

Get a better unit. Can you possibly run cable? Either use a better
grade of video cable, or use a video-over-copper extender if the cable
path is pretty long. Oh, and stay away from cheap video baluns too.

More power?

If you have true line of sight, you are probably ok with the power.

Different
frequency?

Is this really something you can change? I mean more than just a
frequency within a range? If you can change bands, then maybe --- that
usually takes dual-radios etc.

Antenna mounting (I could wear a beanie with it sticking up
from the top - that'd be cool)?

The other poster Wimpie is right on. If you think it's multipath, then
either use antenna diversity by using two antennas (which of course your
radio has to support), or try using a directional antenna with a tight
beamwidth. The stronger (higher dbi gain) antenna will have a smaller
horizontal/vertical beamwidth. And so the chances of multipath will be
lessened.

Keith