Ballistic Missile Submarines
At sea it is even rarer to see a ballistic missile sub than an attack sub, and not just because there are fewer of them. These vessels carry up to two dozen missiles that house upwards of a dozen or more warheads as their payload. The kicker is that they are nuclear warheads. Ballistic missile submarines have one job and one job only: unleash a literal hell upon an opponent in retaliation for launching its own nuclear strike.
Their job, in naval parlance, is deterrence patrols, which are intended to be a simple, lethal insurance policy: if you attack their homeland, no matter how successful you are, a submarine will retaliate by annihilating a hundred or so of your biggest cities with nuclear warheads. Due to this threat, so the logic goes, no one would dare launch a first strike.
If you see one of these at sea, the best indication that you're dealing with a "boomer" (as they're often called) is a sail that is quite far forward or aft relative to the length of the submarine proper, and any disproportionalities in the profile of the vessel. The Delta-class boomer above, for example, appears to have had a parking garage stuck to it. Basically, if it’s a submarine that looks a bit ungainly, or just like one that is carrying a heavy load, it probably is.
And if you do spot one, best pretend you didn't, at least if it is in the open ocean. Boomer patrol areas are, obviously, a closely kept secret. If you do happen across one, congratulations! You've just compromised someone's national security.