Yes, you can get into Robert's Grove (RG) with 5'6". I used to
liveaboard there and Ive piloted two monohulls with similar draft up to RG. I know of at least one vessel with a 7' draft that got in there...local knowledge aboard, high tide, a couple of attempts, but they got in. With 5'6" you will be close to the bottom at times, but you should not run aground unless you get off track and into shoals. The bottom is soft at least if you do run aground.
While there are updated surveys of the Placencia area due to planned
cruise ship traffic, these surveys did not include the back
lagoon. So, there arent any detailed
charts of the back
lagoon, dont rely on any electronic chart there. Best to have local knowledge aboard.
The trickest part is negotiating the entrance to the lagoon. From there N its pretty straight forward, stay well away from banks and points and well out in mid lagoon. The next spot that will likely take a little negotiating is the final approach into RG. The lagoon gets shallower as you approach the banks there. Do not approach at an angle, from well out in the lagoon line up on their channel and come straight in slowly. I have not been in there in a long time so dont know the
current state of their channel.
RG runs a dive shop, so maybe one of their
boat captains can pilot you in on his off time. If not, I can help with local recommendations.
The Moorings provides way points for their
charter guests. I suspect they would share if you asked. That would get you to Laura Beya where their base is located, just S of RG. They could also recommend a local
captain to pilot you in.