I noticed that it's possible to cross over in here: https://i.imgur.com/48uD3aC.png
This is an accepted game mechanic in AtWar. It came about when the original developers provided some flexibility in navigating units through island chains or port cities (for instance around Greece, Indonesia, etc. or London in the case of a Port City) It had the effect of creating land bridges in other areas of the map, so basically wherever a thin strip of land divides water, this can happen.
Note, some map makers have resolved the issue by creating 'no go' areas so this isn't possible. However, in general this is an acceptable mechanic, despite it not being realistic in some cases, such as the one you pointed out.