The hard covers work fine, but the ones I've seen (Impex) are more prone to a bit of leakage and accidental opening than are the best tupperware type hatches. In rough conditions and rescues, I've seen the latches for the hard covers come undone.
Tupperware types range from dreadful to outstanding, but both Valley and KajakSport hatch covers are extremely secure and often airtight.
Canoe / Kayak Anchors
Kayak & Canoe Covers