Even if you live on a river or a lake, you might not want to leave your trailerable boat in the water all the time. Boats left at a mooring or tied to a pier accumulate marine growth, and marine growth wastes fuel and reduces your boat's performance. These reductions in efficiency and increased fuel consumption result in higher boating costs. If you don't want to build a fixed pier or floating dock because of local restrictions or the impact on your boat, you can always dig a slip and build your own ramp.
Measure the length of your boat trailer, from the tip of its tongue to its tail lights and multiply by two. This is how long your ramp must be; few boat ramps are longer than twice the length of a boat trailer. Measure the width of your trailer.
Calculate how large a trench you'll need to dig in order to construct the ramp. Look up the average tidal change (how much the tide goes up and down) in the tide tables for your area. Add this figure to your boat's average draft (how deeply the boat sits in the water). If, for example, the tidal change is 6 inches and your boat's average draft is 1 foot, you need at least 1 foot, 6 inches of water to float your boat. Measure the height of the tallest bunk (the padded supports the boat rests on when trailered) on your trailer: this is the "height of the trailer." Add the height of the trailer to the depth of water needed to float your boat: if the height of the trailer is 3 feet, then 3 feet plus 1 foot, 6 inches equals 4 feet, 6 inches, the minimum depth of water necessary to launch your boat from your trailer.
Rent a backhoe and dig a trench that's wider than your trailer, at least twice as long as your trailer, and as deep as the minimum depth of water needed to launch your boat from its trailer. Dig the trench so that it's perpendicular to the shoreline and separated from the water by a strip of land 8 feet wide, which acts as a cofferdam to separate the water and the trench.
Dump rock into the trench at the end farthest from the water to form a "rock ramp" that slopes downward toward the water. The rock ramp should be at least one boat trailer-length long.
Breach the cofferdam at the end of the trench nearest the water and allow the trench to fill. Pump hydraulic cement over the rocks to form and stabilize the ramp. The concrete will eventually settle and harden.