This is an excellent idea.
If you really want to help and they want you to be on your own (and probably are enjoying their solitude) this is a neat way to do so. They don’t want help now but they may in the future.
Though, as a parent myself, I’m guessing they’d want you to save for yourself and realize they can take care of themselves. Good parents want what’s best for their children without thinking of themselves, it’s a sacrificial relationship.
However, what they do want is a continuing relationship. Call or meet with them regularly. Take pictures of you and your adventures. If you decide to start a family send them plenty of pictures of the grandkids and have dinner with them regularly if you’re close by.
Oh look! A Wikipedia article that answers that.
https://en.wikipedia.org/wiki/Water_distribution_on_Earth?wprov=sfti1#
Of course there’s theories out there that say there’s a lot more water in the earth than we’ve been able to calculate.
Real answer is that our best educated guesses are still that.