No, they do not need to be marine, but construction would add to their lifespan if they will be located where they will receive direct sunlight and water.

Im curious where your current 6 in-boats are located and the amp driving them. Have you done any testing to confirm all the tweeters are working? The 650 has a strong 1" tweeter and this is not a common complaint we hear from a setup with 6 in-boats.