Ive never used it so i cant talk about frequency. I do have plans to collect some. I would just put it on the ground as a mulch. Im lazy. I think its the trace minerals that make it desirable. My local radio garden show promotes it as a spray.
You might look into seawater since you are there.. Water the garden or trees once every year or 2. 1 part seawater to 10 parts water. It has EVERY mineral in it.
I have laid seaweed fresh off the beach 2 inches deep around garden plants as mulch a couple times. It doesn't seem to burn things like lettuce - I do leave about 1 cm free around the base though just in case. The wood bugs seem to love it and it retains moisture nicely -absorbing rain in like a sponge. Slugs dont seem to nest in it, it doesnt blow off or crust over. BUT I haven't noticed any huge fertilizing effect and my sandy soil continues to be sandy. So it's nice but not an answer to all your problems.
To gather it I just wait for a few days of onshore winds then go down to the beach and gather a bit from each pile of it. I try and get a mix of colours and shapes and try to pull from the top of piles where it's dried out and lighter. Personally I don't rinse it before applying but I do water well.
***edit: just checked online, seaweed mulch is somewhere between 1-1-1 and 4-4-4 for n-p-k
I use seaweed on a large scale, being able to get it here at $1.90 a ton, which is fairly cheap. The only problem is, it is very light on nutrients so it takes a LOT of it, about 10 to 1 in terms of PH. I can get Mill Lime for $22 a ton, but despite the difference in price, because I am not paying to moving 10 times more of it, Mill Lime is actually a better deal.
Another problem is how diluted it is. While it has some astounding properties, again it takes so much, that smothering on grass ground can be a problem. In other words, I can only apply so much and not kill the grass, so it is of limited use. Again, Mill Lime is a better deal. Of course where tillage takes place, that is not a problem because a farmer can dump on 10 tons to the acre and just till it in.
On a home gardening scale, I doubt a person could haul enough seaweed from the ocean and hurt their garden because it is so diluted on nutrients. However, it does have some micronutrients that are hard to get otherwise. It is good stuff, it just takes a lot of it!
Observing the seaweed I just put in my garden last week two more things pop out: Texture wise it seems to be synergistic with coffee grounds, if you put both down together the resulting mulch seems to be think enough to block out undesirable growths without getting that weird crust that coffee grounds get. And, 24hr after applying seaweed it is super easy to go along picking all the slugs off of the plant tips as they have fled upwards.
The Greenhouse of the Future ebook by Francis Gendron