Indeed some experts say that otters have sexual intercourse only in open water. Open water in the late winter is very cold. European otters don't delay the implantation of their eggs and can have offspring, and thus also sexual relations, any time of the year. As you may have noticed in these pages the otters I watch live in close proximity with beavers, muskrats and mink. As far as I can tell all the young of those mammals are born later than the otters.
Otters are dependent on fish for food.
Their main compitition for fish are other fish and birds like the osprey and heron. Up here the fish eating birds begin arriving in late March. So perhaps the otters have evolved to have their young early enough after the ponds thaw so that they can get more fish. Then there is that dependence of otters on fish. That confuses me.