Cheapest way to start with ardupilot boat controller

I’m new to Ardupilot, but i want to do my first boat that can do missions. I don’t want to put to much money in to my first project. so i was wondering what can i get on a small budget. i want a flightcontroller with gps it will be controlled from my phone.

Thanks in advance

The cheapest way is certainly not the easiest or quickest way. Expect to have to persevere to get things working.

It is probably best to aim to be able to control a boat manually first.

The boat in the video is probably the cheapest.
It uses quadcopter motors straight underwater with small propellers.

It is possible to control the boat manually from your phone using ESP01 as a receiver and a mobile phone app as a transmitter.
There are various projects ( though I havent tested them) e.g ESP8266 NodeMCU RC car control over WiFi using RoboRemo app - YouTube

Once you have the basic boat working then you can use a ground control system such as qgroundcontrol on your mobile and get a cheap flight controller from a quadcopter

It may be worth it to spend a little more though to make life easier for example by buying a cheap rc transmitter and receiver.