The lake is the being in its individual consciousness, the sea is the same being with a universalised consciousness which can hold the universe and its cosmic forces in itself, the one (individual) merges into the other (the universal). The boat is the formation of the Mother's consciousness in you in which you are preparing to sail in this sea.
It very often happens that when there is an exceptional power in the nature, there is found in the exterior being some contrary element which opens it to a quite opposite influence. It is this that makes the endeavour after a spiritual life so often a difficult struggle: but the existence of this kind of contradiction even in an intense form does not make that life impossible. Doubt, struggle, efforts and failures, lapses, alterations of happy and unhappy or good and bad conditions, states of light and states of darkness are the common lot of human beings. They are not created by yoga or by the effort after perfection; only, in yoga one becomes conscious of their movements and their causes instead of feeling them blindly, and in the end one makes one's way out of them into a clearer and happier consciousness. The ordinary life remains to the last a series of troubles and struggles, but the sadhak of the yoga comes out of the trouble and struggle to a ground of fundamental serenity which superficial disturbances may still touch but cannot destroy, and, finally all disturbance ceases altogether.