The 1st island in Pulau Seribu I visited two years ago.
Only 30 minutes from Jakarta Bay, by boat. Not so far from Jakarta City, but consequently, the water is not as clear as crystal, and the worst thing is the garbage from Jakarta also visited the island, brought by the current.
So far from my imagination of what you called "island". I imagine white sand beach with its crystal clear water where you can swim and easily see small fish in the water.
You are forbidden to swim in this island, I don't know why and I did not ask because even if it is allowed, I won't swim. You'll swim with garbage there... plastic, tin, bottle are easily found in the water.
They provide you a swimming pool, though... (^-^)Y
Beside the unexpected beach, the cottage are quite nice... I chose floating cottage to stay for a night. It's clean and nice...
Not much water activities, but we still can enjoy the breeze and the sound of the wave.
If you want to visit this island, my suggestion is don't forget to bring your PSP, unless you get bored... not recommended island for long vacation (my opinion)
But, still it's one of the nearest places to escape from the crowded city like Jakarta
For the accommodation to the island, you can find local travel agent which will be pleased to assist you, and it will be much more easier for you to use this travel agent service since you'll be offered complete package which include transport, hotel, and meals during your stay.