Have large fleets of destroyers posted around exposed coastal areas covering all sea sectors where enemy subs might try to sneak in for mapping purposes. Usually two fleets of 30 destroyers each are enough. Navigate the whole fleet one sector, stop and look for subs, navigate one more sector, stop and look for subs, and so on, until the fleet has made a circle and is back in its starting position. Sink any subs you find, of course. Thus other allied countries can serve as a buffer zone against enemy countries.
Many players are willing to be good allies.
Until enemy technology gets high enough to launch against any target world-wide, they will often try to get someone to let them have an airbase hidden near your country and launch nuclear strikes from there. I've always been able to find these if I worked at it. Nukers get tired of nuking you if it seems to have no effect. For example, have numerous false capitals, so that when, after great efforts, they manage to find what they think is your capital, and nuke it, only to discover that it was a fake, they will get discouraged. These fake capitals are also very useful as backups in case your real capital is discovered and destroyed.