I am trying to figure out how I lost 1 sector today when I actually grabbed 3. Do ships "claim" the sector that they are on? I did lose a few ships today and was thinking that maybe this was the reason.
Yes. In order to avoid a situation where a country is size zero and still in the game, any water sector occupied by a country's unit (a sea unit or a land unit on a bridge) counts as a sector of that country in the tally.