Moving on a sailboat - bookshelf speakers?

SCM19 and MAC7200

Check out the Usher mini-x with the Diamond tweeter and 7” mid/woofer or a cheaper usher, you could get the x-719 or the s-250, I still have some of these and sold some. 
a nice integrated amp that has the power to power any of these speakers and many others like my smaller Revel’s, is the ps audio sprout 100.

Vandersteen VLR with Van Alstine SET 120 control amplifier would be excellent — I have the VLR-CT paired with same Van Alstine in den system — unfortunate that it won’t fit in your bookshelf.  Used front ported Aerial 5T, used sealed ATC, or a standard MoFi edition sealed Falcon Acoustics LS3/5a ($1995 new) would likely be a good match in your budget. Happy sailing!!!

Your options are extremely limited given the dimensions of the bookshelf. The Gallo Strada 2 seems like it might withstand a marine environment, and I am hard pressed to think of anything better given the limited height of the bookshelf. 

Watkins Stereo Generation 4.  I did not see a budget listed, but these are around $2,500.  Watkins Stereo Store is in my hometown of Kingsport, TN, and have been building speakers since 1974.  Bill Watkins, Sr., founded the company and developed and patented the Dual Bass Driver that was leased and sold to Infinity.  The original WS-1 bookshelf speakers are Holy Grail discoveries in East TN,  Western NC, SW Virginia, etc.  The Gen 4 punch WAY ABOVE their price -- blow speakers that are $10,000 out of the room.