I used to use a 1kg leather Hungarian training ball to warmup my wrists and forearms during dryland.
This ball is much more durable for "impact" training and can be used in the water for legs and passing drills.
All the guys on my Masters team love to pass with it during warmups before scrimmages.
Del Mar WPC Team Store - TURBO Men's Water Polo Suit
Lamorinda WPC Team Store - WPC Silicone Caps - Red