Color Brewer as mentioned, also look into Adobe's Kuler color system. Great tool for playing with color though it only does 5 in a group.http://kuler.adobe.com/
I would definitely color the land fill and maybe the water. Color choice wise I would move the deposits into more muted or pastel colors, light gray on the land and light blue on the water (being careful to keep contrast between the blue geology high or avoiding blue for the geology altogether).
Since you have a 1000m grid showing you might want to line your scale up with the grid. You could drop the tic below the zero mark to keep it on center. And you could drop every other label to reduce the clutter a little. (at this point there may be more text in your scale than on the map itself).
The line for the high water mark looks like an extension of the trail from the parking lot. Try a finer pattern or even a thick solid screened line to emphasize the tidal break. As it is now the outer edge of the colored geology looks like the above tide shore line.
I would like to see the channel label pushed to the left a bit since there is more Chanel on that side of the map. I'd up the pt size 2 or 3 points, use upper lowers and increase the traking to spread the label out a bit.
Not sure how the UK does this but in the US there is usually a set label that goes along with the depoist name (like Qhbd for beach deposits). See if any apply to your map and include them along with the color in both in the legend and the map. Speaking of Beach Deposits, your legend show them as gray due to the legend background.
edited to add: I would also look at stringing the High Water Mark label out a bit. Labels of that sort along a long line can be pulled apart to cover more of the line. Also cartographic convention would be to place the label above the line though I understand that you may want to keep it on the land side of the map in this case.