Reporting back:
The inner tube wouldn't stay tight so I used an exercise leg band ("monster walk") that I had brought along. Wrapped it around twice and it worked great.
First 25y I felt like a jelly fish with my legs hanging down, but it got better from there.
Surprisingly harder than the regular pull sets with no toys (no buoy, no paddles).
I went 1000y with the band (after 600y warmup swim, followed by another 2700y alt paddles/swim); after a while I could pretty much forget I was banded.
I could have gone 2000y; I couldn't have made 4000y banded. It was a good workout.
Interesting thing was swimming after the band workout - my legs felt high in the water. Whether they were actually higher than normal, or just higher than with the band, I don't know.

How much of the band and how much of the paddles should we be doing each week? How long before we should see improvement (as a 1:50-1:55/100y swimmer whose technique has no obvious gross flaws according to various Masters coaches)? <edit>For me the answer is not just swim more - I've just completed my 5th month of 50-55 miles/month. </edit>

Trying to figure out whether I give up on the swimming thing, add this new approach to my existing Masters etc workouts, or focus on this alone.

Thank you.