Slitherine

I've got the free version of Order of Battle and am enjoying it very much. In scenario 3, you can't buy any infantry types, just air and naval. How does one "capture objectives then?

This scenario is a pure air/sea battle. The areas to be conquered are to be reached by ships or by air/sea forces

I guess you're asking about how to capture the "Smalldocks" land location with only boats and planes ?
In this type of situation, it is generally necessary to eliminate any enemy presence to take possession of the premises

Just destroy the coastal gun and then it will change over. Same thing with the Radar.