Traveling to Sulina (Romania) by car

No, you can't, not really. There are some threads on 4x4 enthusiasts' discussion forums on how to get closer to Sulina, but even that depends on the water level.

What's certain is that there's no official road to Sulina, everybody gets there by boat. There are a few cars in Sulina but just for moving things around, from the docks to the shops or getting from the town to the beach.

There's a dirt road between Sulina and Sfantu Gheorghe, but again, you can't reach Sfantu Gheorghe by car, they are both at the ends of channels of the swampy Danube Delta.

Sulina, like other small cities like Sfantu Gheorghe, is at the end of the delta, on the other side of the natural reserve, so there probably won't be a road anytime soon, because of the nature preservation laws.

But the boat trip is very nice, and you won't need your car there anyway. You can leave it in Tulcea without worries – I did 3 years ago for a few days and it was perfectly fine. You have the choice between the slow boat (more scenic and not very long actually) to watch the flora and fauna (birds!) of the delta or the fast hovercraft to get there faster (around half the time needed by the slow boat). Availability of the boats depend on season and bookings.