Prequel: You don't want to transmit without an antenna or dummy load attached; it could break your radio.
If you have a Ham Radio store in town, or Google Ham Radio Outlet, Universal Radio, or other similar suppler, you may find they have something called a "Dummy Load" you can put on your Marine VHF radio for practice sessions. They also showed up on Amazon and eBay.
The dummy load looks like an antenna to the radio - so it's happy - but it doesn't radiate. So you can use it and practice with all the button pushes and screen changes. Of course you can practice just by holding the radio and not pushing the Talk button. But having everything be "real" is better learning. For example, some radios show the remaining battery level when transmitting - a good thing to know.
You can't trust the battery level indicator so much if not transmitting because receiving takes much less current. I remember checking a battery on my portable ham radio at home and it looked okay. So I took it on a long uphill hike with the intention of using it at the summit. Thought there was plenty of juice to receive signals, the radio would die when I tried to transmit - it's like that. So it's important to check the battery level on Transmit before you leave the house; not so much fun to find out on the water.
Things to look for in a dummy load: rated for the frequency of 150MHz or above, the connection matches your radio - usually either BNC or SMA - and its power rating is at or above the level you'll be using.
Getting one with a low power rating is important for cost saving - you only need one rated at 5 to 10 watts. Googling around, I found high priced units that were rated for much more power than necessary. And many Marine VHF radios let you set a lower power level - below 5 Watts.
Note that if the dummy load is rated right at your power level, it isn't designed for long transmissions. Maybe a better bet is it get one rated about 10W (and still use it on low power). I think they were about $15.00 This is something a club could invest in and have radio classes for its members.
Here's a URL for three dummy loads rated at 2 watts for under $3.00 (and free shipping)
https://tinyurl.com/yywoftco
I'd want to do more research (i.e. ask the vendor or a knowledgeable friend) if the unit would work as a VHF dummy load matching your radio antenna connection and transmit power.
Did I mention that you don't want to transmit without an antenna or dummy load attached; it could break your radio.