@freemo to be fair, as someone completely unfamiliar with diving, this is at least a clear and obvious message. I kind-of wonder how many lives are saved each year by sending a clear message like this.
I wouldnt change the sign for a second... if it scares you too much to enter, then you shouldnt be entering.
I have many many hours of hard training to ensure diving caves is relatively safe for me. So when i see that sign it doesnt really scare me, it just tells me what I already know.
If anything it gives me enough of a healthy fear to not loose focus, it reminds me i need to be on my game or die. That is something i should always be aware of.