Cost is $160 per person for an adult and $80 for a child, this is a way to indulge and relax on Christmas Day. (note on their website they state that they require a minimum of 30 passengers for this cruise to operate). Numbers currently booked are not disclosed so you may want to ring them to check.
At this price it is a real 'treat' to yourselves, however you could consider doing this with your family members in leiu of buying each other presents to help justify the costs. Or alternatively since it isn't something you are likely to do every Christmas, just take it in your stride and and go along for the river ride.
I can't think of a much nicer way to spend the Summers Christmas Day, it is definitely worth considering giving away all the usual Christmas chores for a super day out.