Water can absorb about 4,000 times more heat per cubic meter than air, so even small increases in global ocean temperatures could give evidence of global warming. Most available measurements of ocean temperatures have been limited to surface and intermediate-depth waters. Fukasawa el al. (2004) report that deep water across the entire Pacific Ocean basin are warming significantly, suggesting recent accumulation of heat by the entire Pacific water mass.
Reference
Fukusawa, M., and Co-authors, 2004: Bottom water warming in the North Pacific Ocean. Nature, 427, 825-827.