Why don't boats with inboard engines have a multi speed transmissions like in a car?

That is because boats with inboards engines is actually an engine for a car.

actually some do

No need since direct drive one to one can move away the boat easily with out having trouble of overload the engine. as long as the propeller run smoothly with out muddling with any thing under the water that is ok.

There is very much less need for it, and for the very few vessels where there is such a perceived need it is met by having variable pitch propellers.

At normal passage speed (normal sea speed) it is normally unnecessary, and for most boats the requirements are then very adequately met by matching the reduction ration and the propeller (both pitch and diameter) to the requirements of the hull.

At low speed the need is not normally for large torque but the ability to match the engine speed to the speed of the boat, and that is largely accommodated by the natural slippage of the propeller in the water; it grips hard at high speed, but slips modestly at low speed.

The very special requirements for extreme low speed manoeuvring are normally met by keeping the engine at tickover, or not much more, coupled with judicious use of the gear lever; that is an integral part of competent boat handling.

That simple theory does come under strain when punching to windward into a big head sea, or sometimes when towing, or when very heavily laden. If any of these are a major concern then variable pitch propellers are an option - but in a lifelong involvement with the sea I can recollect meeting such a system precisely once. That one vessel was a small warship (the minesweeper HMS Aveley), in the late seventies.

Yeh why? What happens when you get a sloping lake?

They are not needed. Push the throttle, the prop spins, you go.

Unnecessary. The propeller acts like automatic transmission.

The old top gear in a car is known as "direct drive", which means the out put shaft rotated once for every rotation of the input shaft.

On boats the transmission of an inboard engine boat has the same ratio. The selection of the prop is important and any need for more speed can be made there with out increasing the weight of the boat. It requires no more moving parts so it is user friendly. Also if a different drive ratio is desired, the transmission is changed. There are boat transmissions with more then one forward speed.

https://www.powerandmotoryacht.com/maintenance/eve...

the prop 'loses' thrust is spun too fast ...................

They often do have some type of transmission. Regardless they are quite different because boats don't need as much torque a car requires to get going, they mostly just need the higher rpm's. The fluid dynamics of a propeller takes care of the buildup of momentum without the need for a lot of torque.