I live in Idaho so I'm a land locked person here. There's a lake about 30 miles away that has been invaded by some kind of weed. It's some kind of a seaweed like plant.
My question is: would this be considered a sea weed or just a weed?
I know sea weed or kelp is good for the garden or natural fertilizers.

Whether or not it is "seaweed", it is still organic matter that should do the soil good. You could compost it or turn it under. I use the seaweed around here as a mulch. It is mostly a mat-like algae which turns brown and crisp on top while remaining moist and green underneath for weeks. It really keeps everthing moist. I wish there was a way to test it commercially. The water here is more brackish than salty.

If this is a seaweed you can use it as long as in harvesting it you do not also get any algae, which can be toxic, mixed in. If there is also some algae on the lakes surface find out more about it before hauling in the seaweed.

http://desertgardens.suite101.com/article.cfm/what_is_algae_

There is nothing toxic about using algae in your garden. I have even seen blue algae for sale for use in the garden. Pleas show the source for that statement Kimm1

Have to agree with MO here, as I sometimes pull matt algae out of the pond and just throw it up in the flower beds and they haven't suffered at all. Don't have much matt algae anymore since I have lots of plants in the pond, but I will throw old leaves and stuff out and it lands in the flower beds and hasn't harmed anything yet.

M

The only toxic algae is red tide, specifically: Karenia brevis which is toxic to fish and marine birds and if a person were to be exposed to it for a long period it would make you quesy. I prob wouldnt add that to the garden but that is the only toxic one and you would know about it before hand as it would be on the news.