Don't you realize you can't march in a straight line to victory? You fluctuate toward it. From Euclid to Newton there was straight lines. The modern age analyzes the wavers.
Dr. Tamkin tells Tommy this in fourth chapter of the novel, after Tommy has met with his father for breakfast and has been rejected. Tamkin has seen Tommy in the lobby and they have begun to talk, eventually making their way to the market, where they are involved in a joint investment.
The importance of this is that first of all, it illustrates that though Dr. Tamkin may be fraudulent in many ways, he releases truths amidst his fantastic lies or stories. What is important here is that the quote is telling the reader that fluctuation, imbalance, confusion, and unease are all a part of reaching truth. In fact it refers to the idea that Tamkin tells truths in his lies and therefore, reaches the truth in an alternative fashion and not the straight and narrow ways. It also refers to the fact that the "watery grave" on which Tommy seems to be standing, is not a grave but a channel through which he will have to travel to reach his destination, the "reckoning" of his "day of reckoning." Moreover, the symbol of the water itself that is evident throughout the novel not to be the dangerous death that it seems, but in fact it proves to be a life force. In short modern life requires a different path than the past and therefore a different road needs to be paved. Tommy will pave his watery channel with the assistance of Tamkin and the realizations he will eventually reach.