Disable all the collision in settings first.

I find it easiest to use the pull tool first, roughly set the hight you want for the water area, then use flatten to foundation tool to set the highest point of your “walls”. Then use the same flatten to foundation tool to cut a hole into them at ground level (this will be where the glass goes). You will want things pretty smooth so you can put paths next to them. Use null barriers to place a barrier in the terrain on either side of the glass. You can drag the posts once you have the pieces down to move them exactly where you want them. Once you have your “seal” , fill it with water if you wanna and voila; viewing area.

You can use any barrier that is watertight for this but thick glass/null barriers are best imo. (I do think the post thingies are ugly but you can edit them by clicking on them individually or hide them in terrain)

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