Achernar said..
A cheap option, which may turn out to be more permanent than you might think, is to buy a fish-finder with an external puck (depth sounder). Mount the external puck on the inside of the hull with a blob of goo. I used a Lowrance Hook2 4x with a blob of parfix and it works well enough for me to avoid grounding the boat (Cavalier 28). Cost about $195 for the unit and the parfix, if I recall. The advantages, other than cost, are that you don't need to slip the boat to install it, if you find you have installed it in the wrong place, its easy to pull it off and put it somewhere else, and there are no extra holes in the hull.
You need a solid part of the hull to make it work, not core-filled, so check the construction of the Bavaria 33. And, it will not work over the keel or drive shaft, for obvious reasons.
You could use this as a quick-fix while you consider a through-hull transducer, or something more permanent, or more integrated with your navigation systems.
+1 works perfectly inside a solid glass hull