The only way is to heel the boat to one side so that the waterline is above the water. On a sailboat this can be accomplished by securing a line from the top of the mast to an imovable object ashore and winching the line until the boat is heeled to a sufficient angle to be able to work on it. On a powerboat it will be necessary to adjust the ballast from side to side. For example if the boat has multiple fuel and water tanks it may be possible to pump the liquids from one side to the other. Alternatively you may be able to apply weight to one side of the deck untill the boat heels over. On very large boats, these methods may be impractical and you may have to wait untill the next time the boat is hauled or dry docked.
It will be necessary to make sure that this is done on a calm day when the wave splash does not wash the area being cleaned. It may also be necessary to protect the antifouling paint at the waterline as Waterline Stain Remover will attack the antifouling paint and leave it streaky or in some cases remove it. This can be done with masking tape and sheets of plastic.
Once the waterline can be kept out of the water for 1/2 hour or so, remove any loose algae with a scrub brush or power washer. Next, apply the Waterline Stain Remover with a paint roller. Wait for 20 to 30 minutes and rinse clean. In cases where the stain has been on for a long time or is unusually stubborn, a second application may be necessary.
Waterline Stain Remover is biodegradable and breaks down rapidly when it comes into contact with water, but please be resposible when preforming this type of cleaning in the water. Do not apply more than is necessary and try not to spill any directly into the water. After it has preformed it's function, cleaning the waterline stain from your boat, it should be exhausted so that the impact on the water will be minimal.
The best method is still to haul your boat and do it on land if at all possible.