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
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.