As different as cycling and surfing (with swimfins) might seem, there's a way to derive similar fitness benefits from both activities.
This is poignant for me, because we're now living in Eastern Washington -- 5 hours from the coast -- for an indefinite period of time. (Caring for my mother, who's 90.) The obvious way to stay in shape to ride a mat would be to swim with fins at the local Y... and I may do that, especially next summer when the inland heat turns unbearable.
But there is a way to get tremendous benefit from cycling in a manner that relates to using fins. The key is the placement of your saddle. As you raise, lower, and move the saddle back and forth, different muscle groups come into play.
You can't, however, do much about the muscle groups you use when you are swimming with fins...other than changing fins.
So, what I started doing a few months before we moved up here was to kick as hard as I could for as long as I could on a mat. I paid close attention the what areas of my legs started to fatigue first. Then, I repeated the same process riding my bike. I climbed up hills in a seated position putting out as much effort as I could. Again, I noted what area of my legs became fatigued.
I began to move my saddle around until the same muscle group (mostly the outer part of my quads) fatigued first. I rode hard for a few weeks. Then I went for a mat surf. It felt like I had been in the water a couple hours every day...both in leg strength and "wind." Cross training in this manner really worked!
Generally speaking, the further forward and/or lower the saddle, the more effort is transferred to the upper parts of your legs. A higher and/or further back saddle shifts the effort to your lower legs. There's nothing new about these observation...and, this is an oversimplification. Further, it's important that your bike saddle position be comfortable when riding longer distances. There's some fiddling involved, and it take a lot of miles to really get it all fined tuned.
No one, least of all me, is saying that riding a bike is as fun as surfing...but it's a good alternative, with added fitness benefits for anyone who surfs or swims with fins.