Just bought the 903-S6 from Newegg, I'm just wondering if it will be able to be upgraded to water cooling in the future? I'm new to the cooling game so I don't know too much about the requirements for water cooling
Looks to me that you're set for life in terms of watercooling, whether it's custom or an AIO. The front can accommodate a 360mm radiator while the top looks the same though there isn't any mention of the sort of radiator support for that case, I'm only going off of what the mounting holes are and the fans in the case.
I'd advise on scoping out the mounting holes when you have the case in person. It should be good for air cooling as well.
Hard to tell from link as every photo has the front/angle approach and cannot really see the interior of the case. Nearly all YouTube vids focus on similar angles and try to feature the RGB lighting rather than highlight layout, cable management, storage accommodations and accessories.
If by 'water cooling' you mean an AIO, yes, it is fine. If you mean actual watercooling, I would continue shopping to find something that allows for actual pump and reservoir placement as well as adequate space for tubing runs. I'm not saying this case isn't able to support full watercooling, just suggesting that it is very difficult to really tell without seeing in person or with better internal photos to evaluate. Too much focus on the RGB fans and glass, not enough on the actual case itself.