see FAQ Depends on what?

This is a question of Which Water and Bottom Types 

• in excess of 30 m

  • with a blue-green pair of bands in clear oceanic or fresh waters over bright bottoms

• even less than 3-4 m

  • with a green-red pair of bands in highly productive green waters over bright bottoms

• even less over dark bottoms

This is a question of How Much Light

• More photons yield deeper bottom detection

  • June is best in the northern hemisphere: waters still fairly clear, sun at its highest

  • December is best in the southern hemisphere: waters still fairly clear, sun at its highest

• Double the amount of photons entails improvement of penetration depth by ~12%

This is a question of How Much Rain

• you don't want rainy season to come into play whith murky river discharges.