For me personally, nothing beats being outdoors and combining it with a fun activity.
So if you want to get some fresh air, some sun and a wide space, being outdoors is the best option rather than being indoors and being cooped in an enclosed space (indoors) where you get none of these.
As far as being outdoors, for me, nothing beats being in the waters and being under the sun (not all people will agree ofcourse as it depends on your preferences on how you want to spend your time outdoors).
When you combine it with yoga, it somehow changes the whole rhythm in doing the usual easy poses on the mat. Easy and supposedly stable poses such as plank, table top or downward-facing dog, let's say, become more challengingly fun on the board simply because you are in a more dynamic environment (being on a board, water ripple to manage). So it's like revamping the same old yoga practice that you may have been bored in doing since you have mastered them already, into a 'brand-new' practice. You use a lot of your muscles to be able to do easy poses, a lot of core work, arm strength and engagement of your leg muscles. On top of that, you get a nice tan ;)
Maybe like all other things, once you get the hang of it, it might get old once again. I don't know - for me, I've been doing it for quite some time and everytime I get on the board, it's as if I am relearning everything once again as water conditions are not always the same (for my students though, I only pick one safe water condition and would cancel any class if the wind per hour is not as per the recommended practice).
There's nothing more to say about the subject, you have to experience the rest for you to find out what I am talking about :P