I've got a good, easy and flexible setup if you're willing to invest about $500.
I use the Netgear Arlo Pro wireless cameras (now on version 2). They use rechargeable batteries, so they are completely wireless. I bought a kit that includes 3 cameras and a basestation that includes an alarm. The cameras only stream when you view them from the app or when they detect motion. I've found battery life to be 2-3 months, depending on how frequently they are triggered or viewed.
They are completely wireless, so I position them depending on my needs. One stays in the cabin, but I have moved it throughout to keep an eye on leaks or AC temps. One stays in the engine room, which has saved me twice while away from the boat for stuck pumps. I keep one at the helm pointed aft to capture anyone boarding the boat and most of the cockpit. The motion detection is very good with minimal false alarms. The cameras are also night vision capable so you can leave lights off.
On the networking side, I use a semi-fancy router for various reasons, but you can buy a cheaper one for $50 bucks. I also bought a Verizon Mifi modem that I use for connectivity, with marina wifi as a failover. It's an extra 10 bucks a month on my phone bill.
This setup got me through Hurricane Irma when we lost power at the marina, I was able to keep an eye on things until it was restored. I check on the boat frequently and the motion detection alerts me as to when service techs arrive and finish. If you want something more serious for security, you can have the alarm sound when it detects motion.
Some of the big sportfishers in the marina have 6-12 camera setups with DVRs akin to commercial security setups. They cost thousands and IMO don't match this in terms of flexibility and cost.