Limiting yourself to a mask case size, you can still include n extra mouth piece and tie. But no matter how many spare parts you take, if you lack tools to install them, they are of no use, so appropriate tools are n integral part of any kit. I really file that if you can't take"everything" including all part necessary to service a bc and reg, and well as batteries for computers, spare hoses, etc., you really needn't both. You will have to trust tot he boat crew or others anyway. Gear in good repair and maintained regularly is not likely to fail, and preventive maintenance is about what you need for a one day boat excursion. When Debbie and I get away for week of dedicated diving, much of which is done on our own, we take an extensive set of parts and tools and a complete reg/alt air/ low pressure hose/ spg and computer console set up as well, which we can take parts off of if needed, or use in place of our primary gear if we don't want to take time for immediate repair or replacement. However, most places the dive shop will have tools, but not always the part needed, for repairs depending on your gear brand and age and how remote a place you are at. The same is true for liveaboard. However, they usually will have loaner or rental gear you can use if yours fails. Debbie and I just happen to like diving in the stuff we have purchased to suit our individual needs and tastes.
DivemasterDennis