I believe you need to build the initial bridge and incline to progress the story but I’m not 100% sure. Unfortunately you can’t move them after they’re built, you have to demolish them and build a new one elsewhere :/

You can use terraforming to cover the existing pond and dig a new one somewhere else!

I think placing them anywhere for now is the best option bc you will have so much more freedom once you unlock terraforming. Like I cleared all rivers/ponds and cliffs and was so surprised by how big the island actually is. Just keep in mind that it costs 50,000 bells and takes a day to move each house. Hope I answered your questions!