The Washington State Attorney General (see References), ruled that is fully legal for you to collect driftwood from a beach in Washington, with the exception of “logs, piling poles and boom sticks.” Piling poles and boom sticks are both large logs which are used in the construction industry. It is unlikely that these would be mistaken for driftwood. The Seattle Parks and Recreation Service states that "Seattle Municipal Code 18.12.070 prohibits removal of driftwood from any Seattle park." Isolated beaches away from urban areas are therefore acceptable and ideal places to collect driftwood.
Take a large bag with you when you go collecting. This will prevent you having to carry the driftwood in your arms, as you might with firewood, resulting in dirty clothing. Bringing a bag will also help you to carry more driftwood, should you desire.
Choose dry driftwood if possible. Dry driftwood is not only lighter, but you can use it for a greater range of projects. Dry driftwood can be polished and used as an ornament immediately, whereas wet driftwood will need to be cleaned and dried. Clean the driftwood by rinsing it thoroughly with cold fresh water. Polish it when it has dried by using a light-colored wood polish and a clean cloth. Go to the beach at low tide to collect dry driftwood.
Collect the driftwood by boat if necessary. If an area receives a large amount of driftwood, then a small raft or boat will allow you to get closer to the shore but still reach driftwood which is further out. You can then load this into the boat and not have to worry about carrying larger pieces of wood.