When building a boat ramp or a bridge you will have to learn how to pour foundation concrete underwater. The important thing to remember is that while the finished product will be underwater, the construction technically is not. Through the use of a cofferdam you can create a dry space in which to work even in the middle of a river. While constructing a cofferdam for a project such as a bridge is enormously complex and requires special engineering, the principles are the same as creating a cofferdam for a shallow water boat ramp.
Drive a metal stake into the ground on the shore with a sledge hammer as a reference point from which to start all of your measurements. The location of this reference point will be detailed on your print (typically there will be measurements from the property line or other such permanent marker to help you locate where it needs to be placed).
Measure out from the stake to the farthest corner of your foundation. Drive a metal stake in that location making sure that at least one foot of it is sticking above the water level.
Drive two more stakes into the ground, one will be in the water representing the opposite corner to the stake you set in Step 2 and the other will be opposite the reference stake. Measure diagonally between the stakes and make sure that the diagonal measurements (from corner to corner) are the same. Adjust the stakes until they are, this will ensure that your foundation will be squared (have 90 degree angles).
Tie string line between the tops of the stakes so you can see where the walls of your cofferdam will be placed.
Drive stakes into the ground every 2 feet along the string line. These stakes will brace and support the inside of the sheet metal walls of your cofferdam.
Weld your sheet metal into a box, using angle iron to reinforce the outside of the corners. You are going to want your finished box to be large enough to slip over the stakes you have set in place.
Lift the box and lower it over the stakes. The weight of the box will allow it to sink to the bottom of the water bed. Use a sledge hammer to drive the box into the bed at least six inches.
Set up your water pump and begin to pump the water out of the inside of the box. As the water level begins to lower, weld the stakes to the inside of the box. When the water is completely drained from the interior of the box, remove the water pump.
Dig out 4 inches of the mud bottom. Pour in a 1 inch layer of pea gravel and then cover that with a 3 inch layer of hydraulic cement. This forms a "mud slab" which gives you a solid surface to place your foundation forms on. When the mud slab has cured, go onto the next step.
Form your foundation as you would on land. Install any required rebar cage and then pour and finish your concrete as you would normally. When the concrete has cured go on to the next step.
Weld lifting eyes to each corner of the box. Connect your lifting machine to the eyes and pull the box out of the water.