Belgium is ultra flat. You can nearly land everywhere in case of emergency. Every where where there is no building, tree or other obstacle, of course. But ...not all countries are like that.
if your area is very rocky or filled with many height differences, you might have the problem to find landing places. You are counting on your engine to get you to the landing strip and/or you have something that can land very very short.
My club-members tried to convince me that i would love to glide our Bi-Swift in the mountain area in France. I didn't. I didn't even take off. During the fun flight over that area in a motorised open air Dragonfly, i saw nearly no landing places for me. I am a beginner in trying to find thermals to keep our glider in the air. If i would make a mistake, i would need to land between those trees or rocks. No thanks. So ...gliding was not a option for me in mountain area. Maybe later when i am more experienced.
Landing on water is in some areas easier than to find flat areas without obstacles. No wonder that Burt Rutan is working on a waterplane for his own trips during retirement. USA is filled with lakes.
But remember that each country has its own rules. Go see if adding floats to your airplane is allowed as extra in your maximum empty weight. If not, it might be that your ultralight with floats no longer is a ultralight.
Gliding about sea is not really easy. Water is more known as a area where the descent of your glider is larger. You will be quicker on the ground/water if you don't find thermals to climb up or ridges with the right wind to climb higher using ridge soaring technique.
As a beginner i would avoid huge woods if you are not able to stay in the area of your starting place. Finding a landing place is not easy in woods. If you know where they are, you might be in less trouble, but also remember that the wind over trees generates turbulences. Those turbulences can kick your airplane like a play-toy. Keeping it under control might be tricky for a beginner. Especially because those turbulences are most active near the ground. At those heights there is not much room for recovery.
Some areas are very high. The higher you go, the more the air gets thin. Thin air does not carry so good. Your motorised airplane might need more air strip to take off. Might be that you are so high it will not take off at all. I will try to figure out how you can solve this problem. I don't know yet, but i know it IS a problem.