Hi ID10t, I'm so sorry it's taken so long for your showcase to get a look in, exterior reviewers are sadly in great demand. Exteriors are not my forte, but I'll try to point out where your showcase needs work.

The biggest problem I see with your showcase currently is the absence of scaling. You can change the size of statics from 0.5x to 2.0x by holding down 'S' and click-dragging; this lets you get a lot more visual variety out of a single mesh, particularly for plants and rocks.
In particular, scaling the trees can make the island feel a lot less 'flat' and uniform. Have a look at the trees around Seyda Neen; many of them are at 2.0x scale, which lets them fill out the skyline.

Something else you'll need to add for your showcase to be promotion-ready is a pathgrid; the area from the houses and along the docks should be plotted out so that NPCs can navigate it. The pathgrid tutorial in the handbook (http://tamriel-rebuilt.org/content/tutorial-pathgridding-tutorial) explains how to draw pathgrids, but it's relatively straightforward.  (Never mind about this, it doesn't seem like pathgrids are a requirement for exterior showcases)

As Ragox pointed out, some trees are either too far into the ground, or not far enough. I've noted them in the list of specific issues at the bottom of this post.

The islands feel quite flat at the moment, and some parts of the landscape are quite sharp-looking; Morrowind tends to use quite smooth landscapes. In particular, the seafloor around the islands looks quite rough and could do with a bit of smoothing. The seafloor could also do with some detailing; just some rocks/kelp and the like in the nearby areas. 
The elevation change should be a little sharper between the land and water, too; overly-subtle transitions can cause the waterline to look jagged. 
Some of the above-water areas are a little detail-light, too; it would be good to have some small rocks/grass near the well, for instance.

There are some rocks/trees that are still missing vertex shading from their base. There's also large stroke of vertex shading along the southern edge of the main island that should be cleaned up.

There are quite a few places causing texture seams at the moment; Morrowind doesn't allow you to place more than two textures next to one another, so you have to be a bit clever in bordering in any texture transitions you want to make. For simple seams, it's sometimes enough to hide it beneath a rock or tree.

The pond near the houses could perhaps have a terrain_bc_scum ref chucked over it, and a few more lilypads added, to make the area feel a bit more BC-ish.
The 'contain_corpse10_water' should be placed with its back against something, but an undisturbed skeleton out in the open here seems a bit odd; consider replacing it with some scattered furn_bone_* statics.
Bittergreen and wickwheat don't grow wild in the Bitter Coast, so you should replace these plants with something else (and bittergreen vines should be attached to a bittergreen pod rather than used alone).
The ex_de_docks section is not gridsnapped; snappable architecture like this should be assembled with gridsnap on first, and then rotated into position as a group, otherwise getting them to line up correctly is extremely difficult.
The flora_green_lichen should really only be found growing on rocks, I'd replace it with some lilypads.

With regards to cleaning, the best approach is to use TESAME and delete anything other than the "Texture" references and the cells that you've edited. (In the case of your showcase, there are dirty references of type 'GMST' that should be deleted). There's also a tutorial on using TESAME in the handbook (http://www.tamriel-rebuilt.org/content/tutorial-tesame-tutorial)
If you're comfortable using commandline tools, tes3cmd can be used to get rid of these automatically.

Another technical issue with your showcase is it only has dependencies for Morrowind.esm. Your file should depend on the ESMs for Bloodmoon and Tribunal in addition to Morrowind.

Following is a list of floating/bleeding references that should be fixed:

If you need any help, don't hesitate to ask, and apologies again for taking so long!