Submarines?

Yes.  On its next combat move phase, the sub will be in the same sea zone as units from the enemy.  As such, it must make a combat move.  It can either submerge (substall), move away from combat or engage in combat in that sea zone.

SS

To further clarify, all submerged subs resurface near the end of each player’s turn in Phase 8 - Submerged Submarines Resurface/Damaged Battleships Uprighted.  In effect, no submarine is considered submerged at the beginning of any given player’s turn.  This means that a sub that escaped combat by submerging may be attacked again on the next player’s turn (assuming that player is an enemy) and would have to submerge again in order to escape combat.  It also means that it is free to do whatever it wants on its owner’s turn, as Saburo pointed out.