May I wish you a warm welcome to the CameraLabs forum.
The technology you need to avoid over-exposing those water shots is called a neutral density (ND) filter (Wiki article
) but that leaves you a problem as you'll have to source an adapter ring (filter adapter) for your DSC-HX100V, something it's not designed to accept. For one-off shots you might be able to get away with fastening a ND gelatin filter over the lens if you can find a suitable one. Your only other alternative, that I can think of, is to take the shot at twilight or dawn if you are an early riser By the way, check out Gordon's How to blur water for a creative effect
As for the second half of your post, you can't just use arbitrarily short shutter speeds without taking into account the available light. Yes, you can increase the ISO but that carries its own drawback of a potentially noisier (more grainy) image and in any event you eventually hit your camera's ISO limit (3200) after which the auto-exposure system has no choice but to under-expose your images if you insist on using shutter speeds too fast for the available light. Typically you'll only want to use such short shutter speeds if you need to freeze fast moving subjects or you are trying to avoid camera shake at extreme telephoto focal lengths; otherwise they are probably best avoided.