Long Exposure - Water - Tips?

A few thoughts:

What brand of filters are you using? Stacking multiple filters will reduce your sharpness, and stacking lousy ones like those from the discount bin or from China will kill your sharpness. Get a 10-stop ND from B+W, Hoya, or my filter of choice, Heliopan. I can attest that there is no loss of sharpness out of the Heliopans, and I would guess B+W would be just as good. There's no color cast either.

I assume that you're focusing and composing first and then putting the filters on and taking the shot. The problem here is that you could possibly be rotating the lens while putting the filters on which would throw your focusing off. To solve this either go with rectangular filters that slide in a holder (Lee filters) or just use one 10-stop ND screw on filter. Don't put the lens in MF when attaching filters and make sure that the rings don't turn when doing so. This is one of the great benefits of having a lens with internal focusing (IF) because the lens doesn't turn as you focus.

If you have a camera that has a rear AF button, I'd use that instead of using the shutter button to activate the AF. If you're using a zoom lens, and you're pointing it downward or upward, then you can very easily get lens creep during a long exposure. If that's the problem, then bring along some gaffer's tape (not duct tape as it leave residue) and tape your lens down.

Next, are you using either a self-timer on the camera or cable release? Physically pressing the shutter button without a delay will cause camera shake. My way of doing is to: Lock up the mirror, press the button on the cable release, wait a few more seconds for the camera to settle and then press the button on the cable release to take the shot.

Does you camera have a mirror up or mirror lockup option? If so, use it.

Are you using a sturdy tripod? You're using an APS-C DSLR and a kit lens, but you still need a good solid set of sticks especially if you're taking shots when it's windy. Also, make sure that your head isn't creeping during the exposure. If it was, you'd see a smear or streaking instead of an overall softness.

You haven't said what strength filters you are using. I use a 10 stop ND filter.

Stacking more than 2 filters can degrade image quality.

If you are going to stack filters, I'd recommend each be a 6 stop as a bare minimum.

Edit. The AF on the camera will not likely work through the filters properly (too dark), so AF before you attach filters, then switch to MF to stop the focus shifting.

"my photos are soft because I am shooting so closed down" Actually this is a contradiction because closed down should be producing images with considerable depth of focus. and later you say "wider the better for me" which would introduce soft focus. I would suspect you are getting camera blur if you are not using a tripod, which you don't mention.