The fastest route to get there is to take the Taxiboat from Puerto Rico. It is just a 45 minutes drive.
You can walk over to Small Güi Güi Beach (Playa Güi Güi Chico) when the tide goes down. The beach is basically an extension of the Playa Güi Güi Grande. Both of these beaches are about 800 m of black thin sand. Just make sure you keep an eye on the tide if you walk over.
This set of 3 beaches (including that of Playa Tasarte at the far end) are one of the most paradisiac on the island. So making that extra effort will not be in vain!