Water trapped under right side

[CMF_detach8]

After a day of heavy rain I can hear water splashing when driving near/under the right side (driver's side for me). i found my spare tyre in a pool of water and unplugged the drainage holes but that didnt seem to help with water getting trapped under the car

i am guessing the tail lamp seals are bad but why does water get trapped, shouldn't there be drainage holes... i've never seen the car without seats/floorboard so maybe some of you here could give me a heads up on how i can get the water out!