map crashes in game?

every time i go to my map and look behind my self game crashes and its same map is it becoust i got big water area?

do you have -dev -console -allowdebug in the run set up of hammer or -developer 1 in cmd line options
if you do just as you turn you should see a message top right hand side sayying someting like to many vertices ?? if you do then you need to optimie your map