The Pasig River connects the Laguna de Bay to the Manila Bay. This river is not used to transport commerce or anything anymore and is considered dead because of the bad pollution.
Life is unable to sustain in this river. However, the Philippines and many Green organizations are determined to clean up the river and make it sustainable to life. You should not plan a day swimming in the river. There are 13 bridges which cross the Pasig River, as well as the transit railways.
There are many landmarks and historical events which have occurred along the Pasig River. You can see the Malacanang Palace on the north bank, an orphanage on the island and on the south bank is the Sta. Ana Racetrack.
Roxas Boulevard is one of the main roads in Manila you should expect to travel on most of the time. Almost everything is located on Roxas Blvd.
You can expect to find the convention center, trade center, many parks, government buildings, malls, and even the yacht club along Roxas Blvd. This is one of the busiest streets, with eight lanes running north and south.
Most of the main streets run into Roxas Blvd. Once you get to Roxas, you can locate the road taking you to or from the airport, to your hotel and to almost any location in Manila you need to go. It runs along the coast and through the city. It is easy to find your way around the city if you stay on or near Roxas Blvd.