Wind dominates the morphological structure of trees. Weight is not the limit to tree height. Trees could grow many times their current maximum height on a planet with no wind. You could hang iron weights on most trees much greater than the weight of the tree. As long as the wind wasn’t blowing hard, the tree would stand.
Wind, the movement of something as light as air, is able to dominate the form land plants have evolved into.
Now realize that water has a thousand times the density of air.
There’s a reason all it takes to cause a local explosion of diversity in sea life is to dump something solid like a subway car into the sea. Water has so much momentum behind it that life struggles just to remain firmly fixed in one place while subject to waves and current.
EDIT: Also one other major structural factor: soil! Dry well compacted soils are stronger than moist loose soils. The sea bed is an ever-churning bed of loose water logged mud and sand. So not only are ocean plants subject to much greater lateral loads due to the 1000x density of water over air, but those ocean plants also must cling to the worst foundation imaginable.