It is most important to give your body a rest day. At least 1 during your week of working out. You need to let your body recover. I found that even a few days off is good. I was building intensity in my workouts everyday and finding that I was taking longer and longer to warm up because I had the arms-as-logs feeling. I ended up not making it to the pool for a few days and when I got back in the pool it did not take me as much time to warm up and I felt faster.
Taking a break from the pool might help to. Doing free weights can help sculpt your muscles and keep you joints healthy (by building and using your muscles instead of straining your joints).
I found a series of workouts online that has been helpful to me (www.usopenwaterswimming.org). I know, I know, this is still swimming but it will at least change things up in the pool for you.
Plateaus are frustrating. I think you are smart to try and switch things up. See how other workouts feel and then pull your suit back on and dive back into the pool.
Good luck!