I really only use water if I'm going to spit shine the shoe. Then I'm using wax too in addition to polish. As for how often? When they look like they need some upkeep. Otherwise you can just brush them after you wear them.
What are your plans for wearing the suit after the formal? Convention would state black shoes for anything formal especially if it's in the evening. In the long run, black is safe, brown can work though possibly in less instances, again depending on what else you plan to use the suit for.
I have a pair of no-name boatshoes that are my go-tos, and then a Polo pair in navy that are thinner but more comfortable. I wear them with shorts and pants, so maybe you need some slimmer-cut trousers? I think the shape or perceived narrowness of boatshoes is going to be pretty much across the board.