I can't disagree and I think the newer pain would be better diagnosised by what you mean by oversized. I understood it to mean your foot was to small. Not the flippers were long. To cause a tarlo tibial joint or even tarlo navicular joint strain in water might be a stretch (no pun) due to speed and drag issues. Ice and rest should defiantly be tried agreed.
My thought process was that the over size foot hold caused the flippers to via to one side causing and lateral torque for e to the tibialalis anterior tendon. This would fit in with the cramping sensation. Both would hurt when you walk and both would respond to ice and rest.
Ice rest for two weeks and review symptoms.
True diagnosis can only come from palpation and ortho tests.
Otherwise there is the London foot pain clinic which do shock wave therapy great for both conditions.
Happy Holidays