Can I use an HEI
 

I have twin 350's that are getting close to finished ( finaly now that summer is almost over) but I only have 1 single points distributor and need the second. My buddy is trying to talk me into changing to HEI's. I dont know if they are legal in a marine application????

Go with these:
http://www.performancedistributors.com/marinedui.htm

DUI is a great product.

I run it with no problems.

If you are talking about an HEI from an automobile

No they are not legal, not coast guard approved.

If you have an insurance claim it could be denied if they find the distribtor.

DUI as mentioned above is a good product along with many others.

Not to slam DUI, I checked the above link and could not find any information saying they are Coast Guard approved. Stock HEI is not, and it should require cap sealing with spark arrestor vent screens in dstributor housing to vent the ozone buildup under the cap. It says they are marine, but does not say CG approved.

I'm not saying they are not, I do not know, just saying they do not say Coast Guard legal on the web link page above. I may be missing something.

For the record, I do not run a CG approved starter or distributor in my SBC.

If your question was if they are truly CG legal, I'm not sure, phone them up.

I just got one from Dui and it was drilled out on the bottom and had bronze breathers. The cap had no vents and has bronze terminals. All the paperwork that came with it stated CG approved. Also said Marine on the coil inside,but I didn't see anything different from a regular coil other than the sticker. I couldn't run mine because it wouldn't clear any of the thermostat housings I had(BBF). I ended up getting a Marine Mallory unit from Summit. Smaller diameter unit. The Dui looked like a nice unit and I would of liked it beacause of the one wire hookup and no other parts outside of the unit. IE. coil,ballast ect...

Many automotive products could be CG approved if they spend the money for the certification process. Very expensive process to sell to a potentially small market.
I worked at a manufacturer of commercial and marine products for a short while. I asked what the difference was between the CG approved and the other items. He said one has the CG approval tag (and higher price) while the others don't....no difference in manufacture.
Carburetors, ignition systems, fuel pumps, etc have to be made explosion proof. That alone raises the price then add on the CG approval.