Consider the largest land plants, the trees. They need light, so they tend to grow upward to outcompete other trees. However, they also need nutrients and water, which they gather from the soil, through the roots, up along the trunk, and eventually to the growing tips of the branches. Now consider ocean plants. To get light they need to be as close to the surface as possible, but they need no structures to get there, only a means of buoyancy or upward movement.
Nutrients are all around, but at very low concentrations.
In these locations attached plants may have an advantage over phytoplankton because they can sit still and let nutrient-containing water wash over them. In general, on land, the plants are larger than the herbivores. Thus, most land herbivores feed on only a small part of the whole plant, e.g., the leaves, shoots, flowers, roots, etc. In the ocean, herbivores must feed on plants that are very small, so they tend to be small as well. However, oceanic herbivores tend to be larger than the plants they feed on.