I know you realize this, but...
"My husband says no, he doesn't want C taking them out in a boat or canoe, for example, because that's something he wants to do with them."
C doing those things with your kids in no way precludes your husband doing them with your kids too. It's not either or, and the experiences will mean more to them when they're with their dad so it's not like the specialness would erode. Still, I guess he feels like it would be less special if he wasn't first/only in their memories. And yet I imagine he wouldn't mind you taking them out with another relative or platonic friends or a teacher. So it's not really about that.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.