John Masefield's "Sea Fever" is a highly atmospheric poem that conveys the excitement of being at sea. The speaker longs to be out at sea in a boat and mentions several aspects of the seafaring life he misses. At the end of the second stanza, he says,
And all I ask is a windy day with the white clouds flying,
And the flung spray and the blown spume, and the sea-gulls crying.
The "flung spray" here is the mixture of foam and water that is cast up by a rough sea. The "blown spume" is the same white, frothy substance, but blown in the wind.
It is difficult to differentiate between flung spray and blown spume, since the same substance is flung up by the sea and blown by the wind. Perhaps the flung spray is slightly more watery, since it has just been flung up, while the blown spume must first be lifted from the sea (either by the wind or the sea itself) before it is blown.
However, for Masefield, the point of using both phrases is not to differentiate between two types of foam, but to create a blustery seafaring atmosphere
with the accumulation of these details. The speaker is thinking of how refreshing it will be to feel the salt water on his face again and therefore mentions an abundance of it in the description.