Seaweed fertiliser is valuable addition to the organic garden, and is abundantly available for free for those living near the coast. However, caution should be observed when collecting seaweed, particularly from areas that are liable to pollution, such as downriver (including estuaries) of industrial activities, or from coastal areas washed by tidal flows in the vicinity of nuclear power stations, as seaweed is susceptible to radioactive or other contamination.
A perhaps less serious potential problem with seaweed is its salt content. Whilst it is unlikely that you will add sufficient seaweed to seriously upset the balances of salt in your soil, it is not liked by worms, who will not live in it. It can be hosed down before adding to the soil to reduce the salt content.
Seaweed, particularly bladderwrack, kelp or laminaria, can be either applied to the soil as a mulch (although it will tend to break down very quickly) or can be added to the compost heap, where it is an excellent activator. In terms of soil structure it does not add a great deal of bulk, but it's jelly like alginate content helps to bind soil crumbs together, and it contains all soil nutrients (0.3% N, 0.1% P, 1.0% K, plus a full range of trace elements). For those who cannot gather fresh seaweed, it is available commercially in a dried 'meal' form, which can be applied at a rate of 60-100g per square metre, or as a concentrated liquid feed which should be diluted @ 1 part to 15 with water, and can be applied either as a foliar feed or to the root zone.