How to control 12v water pump by arduino? I have arduino uno and I've bought 12 Volt/350 GPH Seaflo Electric Bilge Pump, how to control it directly from arduino?
It's not possible to control it directly. You need relay module (better one is with opto coupler).

Or you can use some N-Mosfet like IRF540 with antiparallel fast diode to supress induced high voltage from coil. However arduino output might be weak for fast charge/discharge mosfets gate and it gets hot.