To be on the safe side, I would recommend having the fuel tank drained and the fuel lines flushed. The water level in the fuel may be minimal but there is no real way of visually checking this as diesel is lighter than water (which means the water will be at the bottom of the fuel tank). If there is water in the tank then it could damage the injectors. Water can also produce bacteria, which forms an unpleasant sludge in the tank that will clog up the fuel filter.

A fuel drain will probably set you back £300 - £400 (or £700 - £800 via your Kia dealer). But it'll be cheaper than risking water damage to the engine and fuel tank.