" 1 Otter ancestor mutates to have webbed feet and takes up fishing.
2 Otter ancestor being adaptable and inquisitive discovers fishing and subsequent selection of mutations make it a more efficient hunter in the water."

3. Otter's ancestors are predators hunting for small mammals, insects, frogs, fish etc. on the shore of a swamp or lake. Shoreline increases in slope. Most individuals limit themselves largely to land. Others are forced by population pressure etc. (because the overall food supply has decreased) to hunt mostly in the water. Those that can swim survive. Those that can't die. The ones that survive pass on whatever genes enabled them to swim better than the others. There is now a split between the land based population and the swimmers, based on habitat. They tend to mate where they find others like them, i.e. around the water or on land, so the differentation between the gene pools is accentuated, the water dwellers now having a selective pressure to make them better swimmers- things like webbed feet, high capacity lungs etc. Eventually the populations drift so far apart that they are regarded as separate species.

So the drive is purely environmental, there's no need to assume pre- adaption or intentionality. I've no idea if this is really how otters developed, what are their nearest relations?