It is

Easiest way I know is to steal an Ambulance, then go to the docks, where you
get Cortez's missions.  Drive up to the 2nd dock (not the one where Cortez's
missions are, the other one), and back the ambulance into the gate, as far
as it will go.  Then get out, and using the hood of the ambulance, jump up
on top, and then run and jump right at the fence.  If you hit the center of
the fence at the highest point in your jump, you should fall through on the
other side.  (I did it on the first attempt, but the next time I tried it
took several attempts to get it right.  YMMV.)  Once you're there, you can
just steal any boat on the dock and sail it to the mainland.

Other techniques that I've heard work are jumping off the bridge on the far
side of the golf course and trying to land on a passing boat, and hijacking
it.  (Dangerous, as you'll end up feeding the fishes if you miss.)

Also, you can steal the Packer from the film studio and use it as a jump.
To do this, use a bike to jump into the film studio over the unique jump
that's there.  Then drive the Packer right up to the gate, get in and out a
couple of times to keep it from disappearing.  Then go around to the
opposite end of the studio, and come back, and a new Packer should spawn.
Drive THIS Packer over the first Packer to get it out.  You can now use this
Packer to jump over the various barriers blocking the bridges.

Yet another technique I've heard of is using the jumping Taxi to cross those
barriers.  This of course requires doing the boring taxi missions first in
order to activate it.

Finally, you can use the Drive-On-Water cheat code to cross the water in any
vehicle, though it's possible that using a cheat code may spoil your chances
to get 100% in the game.  This has yet to be confirmed one way or the other,
but why risk it?

Anyways, once you get over there, however you do it, you should probably buy
the Skumole Shack near the biker bar and save there.  You can then go across
the street to the north of that and steal the police Maverick from the roof
of the station there.  You now have unlimited access to the entire city.