After Shorecrasher Elemental's first ability returns it to the battlefield, it will be a new object with no connection to the Shorecrasher Elemental that left the battlefield. It won't be in combat or have any additional abilities it may have had when it left the battlefield. Any +1/+1 counters on it or Auras attached to it are removed.

You choose whether Shorecrasher Elemental gets +1/-1 or -1/+1 as the last activated ability resolves.

Turning a face-down creature with megamorph face up and putting a +1/+1 counter on it is a special action. It doesn't use the stack and can't be responded to.

If a face-down creature with megamorph is turned face up some other way (for example, if you manifest a card with megamorph and then pay its mana cost to turn it face up), you won't put a +1/+1 counter on it.

Megamorph is a variant of the morph ability. You can find more information on morph on cards with morph from the Khans of Tarkir set.