Doesn't half bad right now, but this can be improved.
It can be done in SbuilderX, but I haven't done it there yet but extensively in ADE, which uses SBX files.
I won't describe step by step (for that you will want to connect with me on Skype (Robert Lacy.... the one in IONE,WA. - I guess there's a lot of us here, but I live in Ione, Wa.)
In ADE use the green icon for polys. Draw a triangle close to the know edge of your airport. (it seems to work a bit better if you have a flatten around your airport. Draw this triangle (one or two apexes - close to the big flatten. ) It helps if you add a number of points to your big flatten all along the edge that faces the water (in your picture). Let us say your airport alt is 40 feet.
when adding a triangle you get a window that allows you to edit individual points (very important to do this).
Now- take the two points of your new triangle and make those altitude the same as your big flatten. Make the other point altitude of your water (maybe it is 0 alt.... fine).
AFTER this is done..... go to one of the points in your big flatten (fairly close to a corner of your triangle).
left click to hilite that point then right click and choose 'copy coordinates'.
Now- choose a point on your triangle close to the big flatten and click then right click and choose 'copy to coordinates'. bingo- That point is now meshed with the point on the big flatten!!!! do this for the next point on the triangle also, but choose the next point on the big flatten to mesh with.
The third point on your triangle which is 0 alt..... leave that one alone. How big for the triangle poly? short or long... you have to just fuss with it and see what it looks like later. You can always readjust the length or size of your triangle. do all this again with a new triangle, but this time use ONE triangle point to mesh with the last, closest point on the big flatten (therefore there are now 3 points meshed together.
And btw- If any rwys, taxiways, aprons, roads etc are too close to the edge that drops down to the water.... you can easily get quick drop offs or cliffs.
As I said - on skype where we can share screens it makes a lot of sense and very easy to understand. Bob