Tropical islands, Berlin, Germany. Inflatable "toys"?

Me and my family are going to Tropical Islands in a few weeks and my three year old wants to bring his inflatable submarine, is that allowed?

I tried to find something on their webpage but I couldn't see anything.

Looking at pictures and videos from the place shows nothing inflatable which leads me to believe that it's not permitted.

Does anyone know for sure if that is the case?