The most important question hasn't been asked: how much do you want to spend?
Pick swimming- usually indoor, low-light mostly action. People spend US$15,000 on camera and lens and are still left wanting. Outdoors, you're not going to do much better on Nikon than a 70-300 outside of upgrading to an 80-400
; even used, that's a sizable jump.
If you wanted to get the best of the best for long standard zooms, Nikon's new 18-140G VR
takes the cake, optically outperforming Canon's also new 18-135 IS STM.
But as also mentioned above, your 70-300 probably does most of what you need, while the D50 will certainly be lacking in resolution and more importantly in AF tracking ability, high ISO performance, and dynamic range, when compared to a more 'modern' camera like the D90, D7000, or even the just-released D5300.