Wander to the beach and start picking up little
rocks (at most you need five, but can carry as many
as seven). Go to the end of the pier, to the right
and down, where you will pick up a long key. Go back
to the wreckage by the pier and click on the bottle
cork to get a clue about a sunken wreck you'll need
to visit later via submarine.

Wander to what looks like another lighthouse (one
of many parallels), probably passing your portal
along the way. Open the lock on the left pillar, use
the long key, then solve the mechanical puzzle to
lower the drawbridge. The puzzle is relatively simple
but for the mouse-click-challenged, here you go:
left, down, down, left, left, left, down, right,
down, right, down, left, and up.

The Roost (exterior)
The tower "roost" has two main rooms,
two lofts, and access to the submarine. For now, you
need only visit the top room and use the submarine.
Visiting the other areas will come later.