A 1 to 2 inch layer of seaweed is commonly spread on a garden in fall and worked into the soil in spring. It is an ingredient in some fertilizers, so it might help your carrots. If seaweed is sitting on top of the soil, not mixed in, it may not have any more moisture-retention benefit than any other mulch. Moisture will still run through sand.
A 1 to 2 inch layer of seaweed is commonly spread on a garden in fall and worked into the soil in spring. It is an ingredient in some fertilizers, so it might help your carrots. If seaweed is sitting on top of the soil, not mixed in, it may not have any more moisture-retention benefit than any other mulch. Moisture will still run through sand.