My last go at this one:

You can spiral outward and keep Kong directly opposite you until you are radius times Mario velocity divided by ten mph from center and then can go straight to coast at shorter distance (r- rv/10) and Mario only needs to go 2.4145 mph. You can go slower if DK doesn't have godlike reflexes but this is minimum regardless of strategy I think.

And that's how simple it is.

PM me your address and size (S,M,L,XL)

If I can make it a little clearer: Imagine that Kong runs 4 times as fast as Mario rows. Mario could row in a circle around the center of the pond with radius R/4 (where R is the radius of the pond) all day, keeping the center directly between himself and Kong, because the circumference of his circle is exactly 1/4 that of the pond. (The circumference of a circle is 2*Pi*R, directly proportional to the radius). Mario can make smaller circles keeping Kong opposite the center even easier, that is, without rowing as fast. Therefore Mario can row in spiral fashion out to a distance of R times (1/N), if Kong runs N times as fast as Mario rows. He can't do that any farther out because Kong would be too fast. But this allows Mario to place himself at leisure 1/Nth of the way to the opposite shore from where Kong currently is. This means Mario must travel R times ((N-1)/N)) in the time that Kong makes the half circle around the pond, distance Pi times R. Doing the algebra reveals that Mario can make it unless Kong runs (Pi + 1) times as fast as Mario. It was given that Kong runs 10 miles per hour, so Mario must row (10/4.14159) or ~2.41 miles per hour.

As I stated in a previous post, this allows Mario to escape no matter what Kong's strategy is.

Later I will post a link to analysis which assumes Kong's strategy and takes advantage of it. It's pretty deep.