So, we have dreams that are non-lucid and we hope to become lucid in them. It's a mental act, yes? You don't realise you're dreaming but you hope to suddenly make the switch to realising you're dreaming. Could it be better if we used a different mindset - where the dichotomy is a visual one instead of a mental one? Here's what I mean:

You would make a rule to yourself that your lucid dreams would only happen under water. So, over time, you begin to associate being lucid with being under water. When you see water in a non-lucid dream, there is a greater chance of you "twigging" when you see the water. Another plus is that swimming under water has a built-in RC because in a lucid dream you can breathe under water. So also, in a way, your lucid dream becomes one long reality check. Over time this would be reinforced - the idea that expanse of water = "aha! Lucid dream"

Another possible positive is that water can exist to some degree in dreams. The mental act of realising you're dreaming is somewhat abrupt and happens in a split second.