My best advice would just try not to be awkward although your first time showering with strangers is going to be.
If you walk in the shower, do your business, and maybe chit chat a little without pulling a peeping tom with other members,
your shower time will be quite enjoyable. Or if you just choose to wash yourself and get out no questions asked that works too.
Just don't be like "Oh man, naked people." because...well...it's drum corps. You'll pretty much be naked at rehearsal.