In Colonisation you transported treasure in a galleon back to the Old World. Therefore a navy became more important.
The implementation of trade route blockades with ships causes you to want a navy more but if you have few coastal cities (e.g. on continents map) this is of little consequence. However since you don't actually earn money blockading harbours the incentive to do it is lessened.
Therefore...
Perhaps instead of automatic trade routes on coastal cities there could be multiple weak merchant ships constantly travelling between coastal cities (not built by you but automatically made by the computer to physically represent trade). Then pirates could keep attacking them and take gold from each one sunk. To counter this a player would need a large navy to protect these weak merchant vessels.
I really like that idea, but I suspect it would be hard to implement well, would add a lot of clutter (and possibly micromanagement), and be awfully hard on a lot of peoples' cpus. If it could be made to work, I would love it.
Naval trade routes are one of those things that have been of immense economic importance throughout history, and which civ-type games have consistently failed to do justice to (except colonisation, but that model is hardly applicable here).
With the somewhat automated blockade system bts has, there's the germ of a useable abstraction - the problem is that unless a city's road links are also all cut off, blockading is not going to do any actual economic damage to the blockaded civ.
If you could blockade naval trade routes and meaningfully impact an enemy's economy (or have the same done to you), the current blockade mechanism could conceivably work quite well as is.
I don't know the nitty-gritty of the trade route programming mechanics, so I don't know if this is a plausible idea, but here's a thought:
Unblockaded coastal cities get a bonus to the value of their existing trade routes - e.g. a larger bonus from the harbour but predicated on not being blockaded.
Extra traderoutes from lighthouses/Great Lighthouse require the city is not blockaded.
Trade routes from coastal city to coastal city get a further bonus provided there is a fully water-borne route for them.
Reintroduce something like the BtS Customs House with a further bonus for intercontinental trade routes.
Thus waterborne traderoutes become significantly more valuable than overland ones, and could provide a significant portion of an empire's income. Cutting off those sealanes could really hit an empire in the hip pocket, and be really worth building a navy for.
Since the largest ports would thus be getting the lion's share of traderoute income (and have the best infrastructure to further capitalise on it), even blockading a single port (without blocking the whole traderoute) could hurt pretty badly.
Likewise it would be something well worth protecting against (particularly if there was more of a barb presence on the seas).
It also makes coastal cities much more worthwhile, which I think is a very good thing (see
this thread).
Obviously, this doesn't account for all the complexities of land/sea/land/sea trade routes etc, but I think it approximates the value-adding effect of merchant marine reasonably well without adding complex new mechanics.
