This is a great little game, and quite addictive!
The first 2 or 3 games I played I got ahead at the start but was later overwhelmed after ~300 turns.
The next game I think I got a "lucky seed" but also I noticed a few more details which seemed to help: the population capacity of the islands (making some more valuable and defensible), "stone" and "timber" islands (presumably give a bonus to production?) and the fact that building a flotilla depletes your population, which needs time to replenish.
I realised with the last part that in the previous games I had weakened my islands by producing flotillas too frequently. With this knowledge in hand I just won in 250 turns!