I spent a couple years running around looking at possible solutions. And here's what I learned:
-wheels won't work at all unless they have crazy suspension like the mars rover or a radio-controlled rock crawler truck
-even with that suspension, tracks will still beat the pants off of wheels but are hard to find and super expensive
-both tracks and super suspension wheels can barely make it up a set of stairs
-anything "innovative" won't work due to power requirements/weight/complexity/cost. That includes legs, whegs, hopping, snakes, and tow cables
-also trying to make it go over water means your project has an 100% chance of failing if you need to ask the questions that you just asked
So you can rule out any wheeled vehicle, even the ones with Mars rover or rock crawler suspension because they can't swim at all. You can also rule out hovercraft because they can't climb. The only real thing left is an amphibious tracked platform (like an amphibious tank). It will cost you $5k-$10k to get a REAL tracked platform which will also be heavy and very difficult to waterproof and make floating. It's actually much cheaper and easier to just build a plane or helicopter.
Trust me on what I say, I looked at EVERYTHING (including snake robots that could both swim and move on the ground).Stick with water or ground. Boats are actually much easier than all-terrain vehicles since all-terrain vehicles that can climb need very complex and expensive mechanics like suspension systems or tracks.