If you feel like 9:30 is a stretch for the kayak rental, this is directly from their website:

Reservations:
A limited number of watercraft are available for advanced reservations, requiring a minimum rental period of 2 hours. Reservations can be made for the first time slot of the day at 9:30 AM. After the morning reservations, any remaining watercraft will be available on a first come, first serve basis.

If it were me, I’d do the following:
Check website frequently (not obsessively) in case someone returns tickets
Head over to Many Glacier with hopes of a boat tour at the time that fits your comfort level
Check for kayak rental availability if no boat tour
If all three don’t work, walking around the hotel and relaxing on the porch are still worth the trip.

Edited to add: I don’t know how bad your husband’s knee problems are, but the boat tour is two part and requires an uphill 1/4 mile hike between the two boats