The space system isn't as well suited for this as you think. And you really need to weigh the pros and cons of having to convert the space system to work with oceanic travel versus writing an oceanic travel system from the ground up.
Well my idea is to simplify the system a great deal. The system I have in mind is merely a method of transportation that will get you across the mediteranean faster than walking or riding with a caravan by land would. There would be public passenger ships that you could take (ie: the pluogus) that are faster than land travel but require you to stop at every port along the way. But more powerful and rich citizens would have the ability to commission their own ships which would be the ultimate form of travel across the mediteranean. That would be the extent of the system. So my main task would be stripping out features rather than adding features.
There's also a metric ton of things that need to be taken into account for sea travel, that don't for space travel. For instance: currents, wind, curvature of the earth, etc. None of these things are even touched upon in the space system, and all of which are extremely important for sea travel, especially in the time period you're looking to create.
That's all fine to think about, but I'm not looking for a highly advanced system here. Though I am intrigued by a mud based on oceanic combat which would have that level of complexity to the system. But that's just not where I'm looking to go with this mud. I do think there should be "random event" type things that could slow your travel down, such as a storm here and there. But that could be handled without all of the calculations of currents and wind and temp and all that.
With the information given as to my intent, does that spark any new ideas for how I could handle this? And thanks for all of the input so far.