Proposal :
Ship Levy Diversification.
Reasoning is that as we have now made land battles more interesting with a diversified levy system that is rather basic, perhaps we could do the same for the naval battles. Not to mention represent different naval traditions in the Westerosi world such as the Ironborn's superior light ships and the Andal heavier warships, or the North's minimal amount of navy(or to mention lack of in canon when a King burned all their ships..).
Ship Types : I believe a simple system of five types of ship should be used.
Light Ships :
-No Combat Bonus, but bonus to surprise attacks and escape rolls.
Examples : Ironborn Longship, Light Galley

Medium Ships :
-No Combat bonus or penalties.
Examples : Andal Standard Galley, Ironborn Larger Longship, Merchant Cog

Large Ships :
-Combat boost, penalty to escape rolls
Examples : War Galley, Merchant Carrack


Huge Ships
-Combat Boost, penalty to escape rolls
Examples : Galleass
Other Types
-Bonus/Penalty depend on ship type.
Examples : Fire Ship, Wildfire Ship, Artillery Ship, Swan Ship, etc.


Levy Composition :
Ironborn :
Due to their tradition of using mostly longships, their levy is mainly Longship.
50% Light Ship, 20% Medium(representing larger longships) 20% Large, and 10% Huge
Andal Kingdoms(Will work out a diversified system later)
Andal
30% Light, 35% Medium, 20% Large, 15% Huge
Purchasing ships..(Edit as for October 18th, refitting for 4.0 proposal)
Light Ship : 10-15,000 Dragons
Medium Ship : 15-20,000 Dragons
Large Ship : 20-25,000 Dragons
Huge Ship : 25-30,000 Dragons
Other : Depends on ship type
Wildfire Ship : 250,000+ Dragons (Why? It's capable of destroying numerous ships in one blast, evidence is in the Battle of the Blackwater)
*Requires some sort of flame ignition
Fire Ship : 100,000
Swan Ship : 30,000 (Better at ranged, good mobility)
Notes :
1. Combat bonus would be +1 for Large and +2 for Huge, or just +1 and both catergories are combined
2. I don't know how much of a naval tradition some of the Seven Kingdoms have, so it's a wip for Kingdoms.
3. Merchant ships may be affected by this.
4. Ironborn and anyone who capture a ship(rolls done perhaps?) can turn it to their use?
5. It's not perfect, I know. But it's a start.
6. Main source : http://awoiaf.westeros.org/index.php/Ships
Edit : 7. Should Fire ships be cheaper(and a one-time use) or remain as is and be considered to be replaced?
Edit 2 : 8.Combat bonus remains +1(for large) and +2(for huge) per every ship, change it to be +1/2 per x amount of ships, or change it to something else entirely?