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In the answer to Can I get fresh water from ocean fish flesh? I learned that the water I can get from eating fish has a low enough salt content that it will hydrate me, BUT the high protein count will cause increased urination to purge urea and leave me dehydrated.
Many land animals get all or most of their water (hydration) from eating plants.
If stranded at sea, can I balance my hydration by eating ocean plants?
Can you keep yourself well hydrated, by consuming only ocean plants?
Theoretically, yes, but you would need to consume a big amount of them.
I'll use the same reasoning as in Can I get fresh water from ocean fish flesh?
If you lose 2 liters of water per day and want to replace 2 liters of water by eating ocean plants, you need to eat, for example, 2,500 g of raw kelp, which, besides water, also contains 40 g of protein and 5.8 g of sodium.
In order to excrete the excessive 5.8 g of sodium (14.7 g salt), your kidneys need to excrete about 420 mL of urine (calculated from Guyton/Hall physiology):
Drinking 1 liter of seawater [35 g salt]...would provide a total sodium chloride intake of 1,200 mOsm. If maximal urine concentration ability is 1,200 mOsm/L, the amount of urine needed to excrete 1,200 mOsm would be...1 liter.
To excrete urea produced from 40 g of protein your kidneys need to excrete 40 x 8 mL = 320 mL of urine (calculated from NAP.edu):
Urea, a major end product of metabolism of dietary proteins and amino acids, requires water for excretion by the kidneys. Renal excretion of 1 g of urea nitrogen (2.2 g of urea) requires 40 to 60 mL of water. Thus, if a person consumes 63 g of protein the volume of water required increases by 0.4 to 0.6 L/day above the basal osmolar excretory requirement of 0.5 and 0.75 L/day in younger and older individuals, respectively.
So, if you get 2,000 mL of water from kelp, which triggers a loss of 740 mL of urine (420 mL due to salt + 320 mL due to urea from protein), you get 1,260 mL of net water, which means consuming kelp can contribute to hydration.
