The highest ( smallest hole ) Aperture is not always the best option.
Depending on the lens your using a factor called " Diffraction " starts from around f/16 and smaller.
Basically without getting to techy, Diffraction kills sharpness..
If you want to know more about " Diffraction " and how it affects lens performance re-sharpness, Just " Google " the subject, Its quite interesting if nothing else.
So thats a first thing to bare in mind.
Next would be your lens, I'm not saying its no good, But it does cover a wide focal range, These type of lens are great and very versatile, But like most jack of all trades, They are not always master of one.
To get the best out of the lens, Try having a look at where it works best, ie: What particular " Aperture " is the one it is sharpest at, You'll probably find its best around f/8, f/9, f/10.
Next " Tripod " for longish exposures, You really need a good sturdy tripod, The slightest movement will translate into blur.
As we are not all rich and have to work to a budget and if you have a fairly light tripod, TRY this little trick, Get a plastic bag or similar, Fill it with around a kilo of sand or pebbles of the beach, Then suspend it from just below your tripods " Centre Column " ...
The added weight straight down will/can in most cases make your tripod much more stable.....Go steady on the weight though....
Next, Shooting tips, Apart from the above mentioned aperture options, Use the " Mirror Lock-Up " option on your camera, This removes any chance of a slight movement as the mirror flips open.
( Its easy to get at on your camera, Its on the shooting mode dial )
Next, Use a release cable or wireless release trigger, This means you fire the shutter without touching the camera, Again this removes any chance of camera induced blur.
Last one for today, But equally important.
When you bought your camera, It should have come with a " Tiny " little plastic thing of rectangular shape.....!!!!!!
This fits into the " Eyepiece " of your viewfinder, What it does is prevent any stray light from entering the camera via the viewfinder, Especially on long exposures, Where your more likely to not be covering the viewfinder with your " Eye or Head " ......
You would be very surprised how much light can potentially affect the exposure in these circumstances, Anywhere from 1 to 2 stops worth, Hence it really screws with your main subject exposure.
Just one more thing.....SHOOT RAW with " Adobe RGB " colour space, This gives you truck loads more scope for processing your image.
Whether or not you use " ND Grad Filters " filters is up to you, They can be very useful when balancing out a scene.....
OK...So that aside your composition in this image is really very good, So try a few of my tips & tricks, Lets see how you get on......