Water sloshing to and fro under acceleration and braking is normally water in the door sills.
Put your head torch on now, get something to kneel on and go and look under the door sill panel that runs along underneath the doors on the outside of the car between the front and rear wheel arches.
You will see three grommets.
If the car is still facing downhill, pull the one out at the front on each side and watch water flow out.
I threw my grommets away several years ago with no adverse effects [a] because I have front mud flaps and [b] because the sill is liberally coated internally with anti-corrosion wax when the car is made.
No, no one really knows why water gets in there!
As for water in the driver's footwell, yes, I would suspect the sunshine roof [if fitted] drain running down the A-pillar on that side is blocked and there are threads about it on here and on www.mytreg.com
[the UK site].
In the meantime, turn the car around and park it uphill!
Also look in the boot under the spare wheel.
Water pools in there if the two small drains up at the top the bodywork [one on each of the outer side] of the tailgate hinges get blocked.
If you have got water in there, park on level ground and drain it by piercing the silver seal covered grommet at the lowest point in the spare wheel well - I used a 2mm drill.