The top of the large boulders offers a great vantage point to spot those interesting marks, supposedly left by the legendary giant known as Bernardio Carpio, and his pets. The other set of marks can be found in Wawa in Montalban, Rizal.
I biked there last weekend (an 81km loop, see related story here) but arrived when the river started to swell and its impossible to cross to the boulders without a raft. Bakas is downriver from two major dams: the Ipo and Angat dams and they release water from these dams usually at around 11 AM. By noon time the current's too strong that they would not even recommend swimming near the rocks.
If you fancy roughing it out here, the shores in front of the Bakas rocks are flat and wide enough to accommodate quite a good number of tents. There are also makeshift cottages (good enough shelter from the sweltering heat in the summer) that the caretaker is renting out for 200 pesos (overnight stay). Amenities: absolutely none!
There are a couple of nice swimming spots further upriver. But current's still a bit strong so its best to stay close to the shore.
The best way is to catch a bus or jeep going to Tungko from Philcoa in Quezon City. From Tungko there are many small buses and jeeps to Norzagaray town proper. Go to the town market and get a tricycle to Sitio Kanyakan where Bakas is. I have not noticed any jeeps going there so you may have to rent the tricycle also for the return trip.