We had an amazing week long stay at Seal Beach in the cottage. Our kids loved the beach, loved the hiking trail and the swings, and overall the property is beautiful. The hosts were very quick to respond, great communicators, and very friendly.
Based on the description it sounds like you are renting a cottage on 22 acres almost exclusively to yourselves, and the one thing that would have helped us prior to showing up was having a better understanding of the property, and the campsites. There are 15+ tent sites on the property - and while they are out of view from within the cottage, it's better to think of this as renting a cottage on a campground vs thinking that you are renting a cottage on a 22 acre property and there is a campsite somewhere completely out of site and mind. Other guests share the property, beach, etc, and on Saturday night the entire place was full. Which, if we had come in with that mindset, our expectations would have been fully aligned with what the experience would be like.
We gave Seal Beach 5 stars and would happily return and recommend it. Our entire family loved it and Mayne Island is a beautiful one with lots to see and explore.