Hello
On your right picture you need to erase the green/ brown rectangle corresponding to QMID grid. "Delete all items" automatically replaced the whole QMID box with "land".
1/ You have to create a polygon following the left vertical side, the horizontal upper side, the right vertical side, and then the bottom line following the coastline (even if not very precise, your lines may overlap the neighbouring blue areas).
2/ Then tag with : Generic Ocean Perennial for instance to rebuild the "blue" part.
3/ After, you can create a new shoreline, rivers, roads, ....
*** Once you draw the poly, you can get the shoreline without drawing line again: select poly / right click: make line; you have a closed line in place of poly; then just delete useless 3 straight sides, and apply tag, width, waves....
Pierrot - FMEP