Kalimantan is bordered by the Sulawesi Sea in the northeast, due south this is Selat Makassar (Makassar Strait). South of the island is the Jawa Sea. The South China Sea separated Kalimantan from mainland Southeastern Asia. Kalimantan doesn't have any good harbours; big parts of the island are barely above sealevel, and the tides influences the hight of the rivers far inland.
Borneo came above the sealevel a few milion yeras ago. By the sometimes high and low water levels during and between the Ice Ages, the island was regularly flooded or dry, and then it was one big island with Jawa and Sumatera and the peninsula Malaysia. Still there are remains of big rivers on the ocean floor which flowed from the mountains in Sumatera and Malaysia to Borneo, the lowest point. On the climax of the last Ice Age, the waterlevel dropped about 150 meters. Borneo and Sulawesi were no more than 40 km from eachother. When the climate warmed up again the ice melted and the sealevel rose; all life moved to the inlands. Now there are identical freshwater fish on Sumatera and Borneo, but they can't be found on Sulawesi.