Many of us like Vedettes du Pont Neuf. They use smaller boats with live English and French commentary. However, you will need to climb stairs to get from the street level down to the river. And also, the best seating is on the upper deck.
If stairs are an issue, you should probably choose a cruise near the Eiffel Tower area a taxi can bring you closer to the boarding ramp.
Although limited free reservations can be made 2 days in advance, you do not have to have a ticket to enter the Cathedral of Notre Dame. Usually, the wait is less than 15-20 minutes. Admission is free.
Does your husband use a cane? You will find in Paris that using a cane or other mobility device will allow you special access to most venues. Here's the information for
accessibility at Notre Dame.
Another possibility would be to attend a service at Notre Dame. There are several throughout the day, plenty of seating and you could look around the Cathedral afterwards. There is a separate entrance for those attending services.