I'm not the great professor (because he truly knows the game like the back of his hand), but I can tell you that you'll come back through here later once you have the gas mask, and the water will be back down to its normal level. So don't worry about it for now. In fact there are several places you might reach in the game that require the the gas mask before you actually acquire it.
The game can be very frustrating until you stop trying to second-guess everything. You'll feel like this many times... ....but as a rule, if you encounter something saying you need an item you don't have (say, a Torch or a Gas Mask) it means you'll be coming back later, so there is no need to worry...even about underwater areas..... (Note that there are some items you can buy from the Shaman, like the Breakdance Strike, that are not REQUIRED. The items that you MUST have are ones you will find in turn.)
BTW, it might chuff you to know that if you got to the Great Tree, and saw it filling with water, you're going the right way. If you have young kids in your house, though, I suggest you put them to bed and stick earplugs in their ears before you attempt the next level, unless you don't mind them hearing the more vulgar parts of your vocabulary.
I know that i asked this before but since the other stuff was lost i'll ask again...
It says that it includes the classic version of pitfall and pitfallII?? whrere or what or how is it??
To unlock Pitfall II, you need to find all the Idols and Explorers and buy every item from the Shaman (meaning you also need to find every Shaman, of course). Note that to get all the Idols, you'll need to play the original Pitfall.
The first time I went through the game I think I bought that item from the Shaman in White Valley because I didn't have enough Idols at the time to buy anything else. Don't remember where I bought it the second time.