We need a feature for ships and land units to form a blockade. Right now someone can sneak past all my maritime defines lines; land a hundred troops and than start a war. That's very unrealistic.
Not familiar enough with S1 to give concrete answer; I wonder does S1 have High Command feature?
With S1914 High Command gives you fire control option of Aggressive Fire Control which will start WAR with any unit crossing its path, so positioning ships at weigh points on the water will prevent any other units from crossing the weigh point and there by creating a Naval Blockade.
Interestingly enough, fire control is available to all players; you don’t need High Command to access them (unless this is not true, then I have a problem), so the naval blockade strategy that Hastings suggested is very much doable:
1. Place ships in ocean around nation
2. Set fire control to aggressive
agressive is a stab on the back if you use in most games, you will declare wars on IA's (IA sometimes randon send conscripts on the ocean so you will declare war like to portugal if you are trying to block england)
even neutral players by accident, the most safe way is set fire mode to Offensive (not agressive this is important) and declare war on the ones who you want to block
unless you are defending your coast, soo agressive work to defense.