float tubing always destroyed my weak knee (patella tendon issues). eventually i switched to a outcast commander so i can row around and move spots faster. its not as good on anchor, but overall i just find it better.
id say the best exercise to simulate it would be one of these things i think
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During the warmer months I do enough outdoors fun stuff and work around the property to stay pretty fit.
During the colder months I do strength training; working a different muscle group + cardio @ home 3-5 times a week depending on my surprisingly busy retiree schedule. I spend about 1.5 hour total each day, that includes the tracking "paperwork".
For strength training during I have a set of adjustable 10lb-50lb dumbbells on a 2'x2' "rack" stand, a folding weight bench, and a stability ball.
I do 6 sets totaling 64 reps; increasing weight while decreasing reps for the first two sets, two sets are at the heaviest weight and effort, then work my way back the other way for two sets. The first and last sets are at a low enough weight that it's more like stretching.
Cardio: For a year since we got our 4 year old rescue Lab - Shep mix I follow up strength workouts with taking him on brisk-paced 2 mile walks.
I had a birth defect in my legs and am told I wore braces for about 4 years. I needed a hip replacement at 55 due to a deformity caused by the braces. My ortho surgeon told me if I wasn't a runner not to start, and to use trekking poles when hiking & backpacking to protect the prosthetic from wear-tear; after 6 years there was no wear - it's working. So I have a folding low impact cardio machine similar to a ski machine to do interval cardio that's real good to train for a backpack trip or I will combine "leg days" and cardio in the warmer months in 45 minute road rides on my MTB as fast as I can sustain near my home. I also have a free Silver & Fit Y membership and use the fitness coaches to critique and refine my home workouts.
I find hamstring curls on a stability ball do a great job at strengthening hamstrings (and knees) for float tubing and wading. Stability balls are cheap and don't take up a lot of room. A chair may also work.
I'm at the point where I can do 90 reps in 6 sets once a week. When my legs are strong enough, I switch to alternating single legs in the sets.
Don't forget adductors for wading. I use a heavy resistance band secured to my weight rack for standing adductor exercises. Again 90 reps over 6 sets steadying myself against a door or wall.