It depends on a couple of things. One is how far you want to go with your bridge and if there are rocks and boulders that you can use as attachment points. You can also time the waves and use the hoe to raise terrain in shallow water to create attachment points where there are none.

The very first game world I played in I had to do that as my start island was pretty small and I had mined every thing out on it before I completed bronze gear. There was a rather large island across a narrow channel that I was able to build a bridge to. I built workbenches on the rocks and use wooden floors to cross the gap.

Of course that island was infested with trolls and I hadn't yet found enough spare cores to build 2 portals but that's a different story.