I want to insulate the engine compartment in my boat. Ive read about sealing up all the air leaks out of the compartment, but want to know what products other people have used. Was there a product you used that was a PITA to install or did not produce the results you wanted. I've read about Dynamat but a lot of people say that there are other products that produce the same effect at a fraction of the cost.

Dynamite is overpriced sound deadener that is flexible to line the wavy surfaces of a car floor-trunk. My hatteras has flat stuff that looks a lot like flat foam insulation from home depot. Way cheaper and you basically need 2 flat pieces. Homedepot.com and put rigid insulation in search block. There are some with foil cover on 1 side, but the stuff is soft and will get gouged and beat up.

I was thinking about adding some sound deadening to my engine compartment too. In regards to the HD approach, I think you need some type of flame retardness for a marine application?

Do keep in mind the engine has to breath. The factory rarely has enough openings for the engine to breath properly.
My insulation must be working great - I can't hear my engine at all.
Open exhaust tends to do that.

I would hate to not hear a lifter going, unusual and abnormal sounds or detonation getting up on plane.
That's just me though.

I prefer to hear the mechanical nature of all machines.

"Sounddown" 2" foam is AWESOME at killing medium to high frequency sound.
I have Caterpillars in a sport cruiser and while I love turbo whine and mechanical noise, it was vicious on that boat. Added SoundDown across the entire transom, some on the sidewalls and fuel tanks, and some on the front bulkhead - amazing reduction in clatter.
The most effective, though, was that the cockpit floor was comprised of removable hatches (above the Cats) and had PARTIAL snap-in carpeting. It was good, rubber backed carpet, but it did not cover the ENTIRE floor. Added snap-in pieces to cover ALL of the floor/hatches and all of a sudden, the only noise was a satisfying and muted whine out of the waterline-mounted exhaust bypasses (main exhaust was large underwater bullets which gave a low rumble from the rear of the boat when at planing speeds).

BUT - you DO have to get plenty of air to the motors. I used some SoundDown on the air intake reflective surfaces but not in a way that obstructed airflow.

By the way, I also used this foam to make "sound catchers" to kill the noise of the AC compressors inside the cabin.

It is the same foam Peterbilt uses on their trucks.