The leading thought in today's liturgy is that of trust in God in the midst of struggles and trials.
This thought springs from the reading of an incident in the life of St. Peter. The Gospel was chosen due to the near approach of the feast of Saints Peter and Paul kept on the twenty-ninth of June; a Gospel book (Evangeliarium) of Wurtzburg actually calls this Dominica ante natalem Apostolorum (Sunday before the heavenly birthday of the Apostles).
It was from St. Peter's boat that our Lord chose to preach; it was Simon Peter that He told to launch out into the deep, and it was he who, at the Master's word of command, laid down the nets which became so full that they broke. Finally, it was St. Peter who overcome with astonishment and fear, adored His Master and was chosen by Him as a fisher of men.
St. Ambrose tells us:
St. Matthew describes this boat as tossed by the waves, while St. Luke describes it as full of fish; here we have a picture of the Church's vicissitudes in her early days and of her wonderful prosperity later on. The vessel which carries divine Wisdom and which is wafted by the wind of Faith runs no danger.
St. Gregory writes:
What does the sea represent, if not the present age in which the changes and chances of this mortal life are like waves which unceasingly dash and break against each other? Of what is the firm ground of the shore a figure if not the permanence of eternal rest? Because the disciples were still surrounded by the waves of this mortal life, they toiled on the sea; and as our Redeemer had put off the corruptibility of the flesh after His resurrection, He stood on the shore.
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.