Before deciding on the model or price range to go for.. perhaps its best to get an idea of the capabilities and the functions on fish finders so you can better choose one that will suit your fishing requirements. Any fish finder can be used in an inflatable boat.
Example..the Plastimo 300 will not see the bottom of Loch Etive never mind find a fish. (Loch Etive is one of Scotlands most popular sea fishing lochs )
Its too deep.. it goes down to 400ft at places and is sea water.
From the spec of the Plastimo 300..it will show the bottom at 150ft ..although I suspect this will be in fresh water and not salt water..I also doubt if there will be any definition at that depth ..but having never used one ..I could be wrong.
Here is a good web page that tells you about the different things to look for..so you can match them to your fishing requirements..
Read it and you will be in a better position to decide the best type for you.
How To Choose A Fish Finder | Fish Finder Source