Peter Beck (Rocket Lab CEO) has explained in a few interviews that maintaining a fleet of boats is incredibly expensive, and that this cost is one of the major reasons why they didn't go this route. Operating helicopters is much cheaper in comparison.
He has quoted actual numbers in the past, such as in this December 2021 interview (https://youtu.be/gcuOSXjevGs?t=751): "Do you have any idea what it costs to operate a ship? It's way way cheaper for Electron to fly it with a helicopter than it is to put a ship out there for a couple of days. It's just ridiculous. Marine assets are SO expensive. […] The Bell 429 that we operate is like $3,000 an hour. […] It's a 2.5, 3 hours flight. […] It's like $60-70,000 for the ship just to sit in the port! Not even going anywhere, it's $60,000 a day. And that's just a tiny little ship."