Very valid point, but I would like to say that if you are faced with both situations and only have the budget for one set of grads, the soft grads will (IMHO) work better on water than the hard grads on the rolling hills with forest scenario.
I think this depends a lot on when you are photographing. I spend a lot of time photographing at the coast (it is my back yard) and cannot recall ever using a soft grad; with low sun soft grads give me very little benefit because the transition is too gradual to help with brightest area, which is often close to the horizon, particularly at sunrise/sunset (even here in the frozen north of the UK the sun does occasionally penetrate). I occasionally use the soft grad when photographing mountains, but I find I often don't need a grad because the contrast is not as extreme.
for my needs are soft grads preferable - as far as I know there is no see within 600km from where I live, but there is lot of great mountains