S-Hawk's fruit is just to compensate the fact he is using a mere replica of Mihawk's sword. Which means he has a less quality blade that could be destroyed during combat. Having the blade fruit would diminish this risk by giving free access to a blade anywhere.
But S-Shark's fruit perfectly compliments his fishman abilities, by giving him access to "water-like environment" without being weakened if he were to submerge in actual sea water.