Meet Halosaccion glandiforme. This seaweed is a bit like a water balloon — an inflated oblong sac partially filled with water.
Halosaccion is streamlined and flexible — designed to withstand currents and waves. It also has tiny pores near the rounded, distal end (away from where it attaches to the rock). The tiny pores allow water to flow into the sac when the algae is submerged under water. Then when exposed to air during low tide, the water inside the sac helps prevents desiccation (drying out), and might also play a role in maintaining a lower temperature.
If you find Halosaccion washed ashore, you can see evidence of the pores for yourself. Compress one of the sacs between your thumb and finger, and watch powerful jets of water stream out:
Some of the information above is from the following paper: Vogel, S. and C. Loudin. 1985. Fluid mechanics of the thallus of the intertidal red alga, Halosaccion glandiforme. Biological Bulletin 168: 161-174.