Get some Locktite and put a bit on the threads of the waterbottle screws and it should minimize this. Not cheap but useful stuff. Highly recommended for cleat screws and jockey wheel screws also. A set of small metric wrenches, basically 8, 10 and 12mm cover almost all the nuts. A 2mm, 4mm, 5mm and 8mm hex allen wrenches get all the allen screws. You are up to maybe $15 at this point. Add a pedal wrench, which doubles for the wrench needed to torque your BB remover tool and that is about it. A few bikes have 6mm allen
sockets and my old Cinelli stem needs an odd ball 7mm. That covers the basics. Adding a casette and BB removal tools, a crank puller and chain tool, and chain whip
and voila you have a tool kit. The under seat bike bag should include a 5mm
allen wrench as it covers most of the bolts on Shimano groups. Get the basics, get used to checking the bolts yourself and the bike will be that much less mysterious. steve