Sirens counter amphbots pretty reliably until Grizzlies are fielded.
Sirens outrange Scallops, and if they are thrown against the Sirens, they will be bunched up and suffer greatly against the AoE, while the Sirens can move away and start outranging the Scallops again.
Ships easily beat amphs on open water.
However, in more cramped situations, Sirens will suffer against enemy artillery such as Envoys and Lance, and Grizzlies.
https://zero-k.info/Battles/Detail/888519In this game, my plan was to make mostly just Scallops, but I found that they would only work in defense as they were too quickly shot down by Sirens, Envoys and Torpedo Launchers.
Throwing them against all that AoE of those units with a Lobster, would have been suicide. They were still a good defense though, as then they would only need to surface to deal with incoming units and could remain underwater safely from artillery the rest of the time.