the rangers recommend a safe distance of 30 feet. I get no closer than 10 to one on land, that gives me enough time to haul my old ass out of there if i happen to piss it off. if they get disturbed they usually just slide into the water and submerge or swim away.
If you happen to get near a nest they'll get aggressive.
As far as the park goes, you don't need a guide. it is set up with the public in mind. the beauty of it is the size. It has gravel covered walking trails, around two lakes. A complete loop is 6 miles.
A well maintained habitat.