When we affirm that "salvation is by grace alone" we are declaring that grace is not a reward for faith, but the cause of it. That Jesus is not only necessary, but sufficient to save us to the uttermost. That is, He provides everything we need to be saved, including a new heart to believe. Have you ever heard a synergist respond to this by asserting that faith is like a drowning man reaching out his hand to someone in a boat and thus faith has no merit in itself. Here are a couple of things I like to say in response to this:
If you liken the sinner to a drowning man reaching out his hand to God and claim that this needs no merit from which to boast, consider this: You appear to assume from this analogy that the drowning man (the sinner) believes he is drowning (believes he is a sinner) and is actually humble enough to recognize his own plight. But we must ask, are there any sinners who are naturally willing to receive the humbling terms of the gospel? Isn't it grace itself that makes us humble? Isn't it grace that makes us recognize we need Christ to save us in the first place? So then, do you believe that some are saved and not others because some are more humble by nature? Do they naturally recognize their wretched condition and need for Christ, apart from grace?
Let us also consider what true love for a son really looks like in a drowing situation. When God sees us "drowning" He does not merely reach out his hand hoping we will reach out ours. We are beyond that. We can't swim. We are unconscious; totally dead to Christ (1 Cor 2:14; Eph 2:1). Rather, when He sees His children in trouble, True Love, not only reaches a hand, but jumps in the water to make certain we are safe (Isa 55:10, 11 & James 1:17-18). What loving parent would do otherwise? If God were to require us to do our part in swimming. reaching and resuscitating ourselves, then His love for us would be conditional. If any of us witnessed our neighbor's parent letting their child drown this way, we would think it unconscionable.
Again, what kind of parent would merely reach out His hand to save someone who was drowning and not offer further help if the child could not reach out to him? What kind of love it that? Your parent analogy sees his child in trouble and will only save him on condition that he has the capacity to swim through the waves and reach out and take hold of the father. The father will not, however, risk his life to actually MAKE SURE that the son does not drown, if the child is unable or unwilling to reach out. His love does not act, so it is an ineffectual love. His love depends completely on how the son responds. This means his love is not the unconditional love the Bible speaks of. Frankly, most people understand that the true love of a parent would unhesitatingly "violate" their sons will if it meant it would save a child from drowning - because the parent knows better than the child what is good for him. His love is not weak-willed or ineffectual but he loves his children with a resolute will that accomplishes what His love dictates by actually saving his child, even by forfeiting his own life in the process. Again, is a father who MERELY reaches out his hand to a drowning son, then does nothing more, a loving father in any sense of the word?
What about those people God did not save?, Jonathan Edwards once wisely said, "If damnation be justice, then mercy may choose its own object." By using "drowning in a lake" as an analogy, you are making it sound like our condition before God is innocuous. This logical fallacy is called an "appeal to pity" (ad misercordiam). Perhaps if our problem were only of a physical disability or of an innocent man drowning then of course we might be more inclined to make God out to be an ogre if He chose not to save him. But this is not how the Scripture describes the disposition of a sinner's heart. The Scripture says the unregenerate are rebels, hostile to God by nature (Rom 8:7). Realizing that analogies are imperfect, this drowning analogy still depends on pity for it to work at all and is actually imposing an alien presupposition on the Scripture that we were just helplessly, innocently in need and God is, therefore, obligated to reach out to save us, lest we drown. So according to this analogy the one condition we must meet if God is to love us is to reach out and take hold of His hand which He is also obligated to extend. I hope you see the clear problem with this reasoning. The Bible plainly teaches that God would be perfectly just if he saved no one. Any good gift we have is better than we deserve, and is granted by sheer mercy, in spite of ourselves. We are not His children by nature (Eph 2:3) but are adopted as children by grace. God is in no way obligated to to cancel anyone's debt, but because He is loving and merciful Jesus paid the debt by absorbing the full wrath of God for those He came to save and applied salvation to them according to His sovereign good pleasure (Eph 1:4, 5). To those who are His children (Rom 9:7-8; Gen 21:10), He will do whatever it takes to make sure they are delivered from the jaws of death.
We are all naturally self-righteous. The grace applied by the Spirit alone humbles our hearts to see the righteousness of Christ. Humility is not drawn from our native resources, but is itself the gift of grace. The humbling terms of the gospel are received only by the brokenhearted, that is, those in whom Spirit has wrought faith.
It is true that faith has no merit in itself when it looks away from self to Christ alone for salvation. But if we believe this faith came, not as a result of grace, but is something we came up with from the unregenerate self nature, apart from grace, we are boasting. Such a person could thank God for all they have, except faith, the one contribution they believe they made to their own salvation. But the Bible teaches, rather, that Christ provides all we need to be saved, including a renewed heart that believes.