Argon vapour very much behind source of some marine obstruction (8)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

AR (Ar = chemical symbol for argon) + GAS (vapour) + SO (very much, as in “I so wish that . . .”), all behind the first letter (source) of S [ome]. Type of seaweed that forms large floating masses, especially in the region of the Atlantic known as the Sargasso Sea.