Thanks for the question!
It’s true. Fresh spinach retains more water than frozen spinach. Part of the reason for that is because frozen spinach is often blanched before it undergoes freezing. Blanching helps kill harmful bacteria and reduces loss of flavor, color and texture. During this process though, the spinach does lose some water. With that in mind, freshly blanched spinach and frozen spinach is a better comparison. In this case, the water content is not significantly different and is about the same.
One advantage frozen spinach has over fresh spinach is that it retains more nutrients and folate. Though fresh spinach initially has more nutrients than its frozen counterpart, it begins to lose these nutrients as it is transported from the farm to the market. During this time, fresh spinach loses enough nutrients that frozen spinach ends up having more. This is because the spinach is blanched and frozen shortly after harvesting which essentially freezes it in time.
As can be seen, both fresh and frozen spinach have their benefits. When debating on which type to use, it really just depends on what you plan on cooking and what will work better for that recipe!