The oil mixture depends on the age of the watercraft and type of oil being used. See the table and comments below.

Ratio	Oil Mixture	Comments
32:1	4 oz oil per gallon fuel	This mixture should be used when breaking in new engines. The length of the break-in period will be specified by the manufacturer.
40:1	3.5 oz oil per gallon fuel	This mixture ratio was common for older watercraft that used TCW and TCW2 oils. It's really not necessary when using TCW3 oils.
50:1	2.5 oz oil per gallon fuel	This is the normal mixture ratio for premix engines. This ratio may also be used for a watercraft that are within the break-in period if used in addition to an oil injection system.