San Baada is walking distance to Super Joseth Express which is a supermarket right across the street from the beach right near Marlin's Resturant. You can get all the food and drinks you would need - though the prices will be a bit more than the various supermarkets in Quepos.
The main branch of Super Joseth is closer to Si Como No.
The way the Manuel Antonio is set up, there is a string of hotels along a 7 km road between Quepos and the park area and also some on roads branching off from the main road. . There are various restaurants and hotels right in Quepos and then along the road in various areas, there are some pockets with several restaurants fairly close to each other.. There are some close to Si Como No. And in the beach area near San Baada there are also some. So if you stay at either of the two places, you will be able to walk to some places and then either take a cab or bus if you want to try some of the others.
Inca has given you a good idea of the perils of walking in much of the area.