For sand / ankle-deep water I use a large waterproof sock: SuMade 100% Waterproof Socks, Breathable Knee High Cushioned Hiking Cycling Skiing Kayaking Socks 1 Pair https://www.amazon.com/dp/B08KW7CNR8/ref=cm_sw_r_cp_api_glt_i_39N5S37G7DC3RZJJ1XKG?_encoding=UTF8&psc=1

For actually submerging/getting it wet, I use what I refer to as a ‘leg condom’, it’s air-tight and holds up fairly well, although not as good as a water-proof prosthetic: Waterproof Prosthetic Leg Cover - Airtight & Submersible - Ideal for Swimming or Shower - Large (PL-17) https://www.amazon.com/dp/B003XFSHUE/ref=cm_sw_r_cp_api_glt_i_D1CPQVKT21B754HZR5HW?_encoding=UTF8&psc=1