If you are somewhat happy with the location and just want to add a little since it sounds like your current speakers are low in the boat you could do something like these.
http://www.polkaudio.com/products/db6501Depends on the size you have but I put these in my last boat and at just over $100(amazon) for a set they sounded great. I mounted the woofers under the two consoles and i just double sided taped the tweeters just above the gunnel opening so you couldn't see the wire. The tweeters can be flush or surface mounted so you could also just use 2 screws to secure and drill a hole for the wire. They really add to the sound since the low speakers will loose all the highs and being so small can be mounted almost anywhere. I only had a sony head unit so they don't require a ton of power.
They also have some other sizes like the 3.5" ones and other shallow flush/surface mounts you could mount almost anywhere but may require more power if you already have 4 speakers.
Keep me posted. I would like to change my current setup. I can't hear anything from the drivers seat unless I crank it up on my v258 since the 2 cockpit speakers are mounted so far aft.