They are my favourite civ.
All of their Unique Ability/Unit/impovement is very powerful.
UA allows you to have 6 gpt per ressource. You can trade them without to be worried about happiness.
And then when you etablish a trade route with a civ, there is a big chance to get them like ally and next you can get 240 gold per ressource.
Polder makes your city very tall
and you can put your city in the middle of marsh => It's very good defensive position.
Sea beggar built in a city with barracks and armory can have +60% against city + double attack + heal everywhere + steal naval unit.
So you take 6 sea beggar and they can take a city easily, when a beggar attack, the others heal. And for that you earn money !
Then when you upgrade them to destroyer and you are the master of the water, because they still have the ability to steal other naval unit.
PS : Sorry for my english