In this letter Walton seems to be really feeling the loneliness of being on an isolating sea voyage.
In this letter to his sister, Walton writes:
"I have no friend, Margaret: when I am glowing with the enthusiasm of success, there will be none to participate in my joy; if I am assailed by disappointment, no one will endeavor to sustain me in dejection. I shall commit my thoughts to paper, it is true; but that is a poor medium for the communication of feeling. I desire the company of a man who could sympathize with me; whose eyes could reply to mine."
We learned in the first letter to his sister that he initially began this "undertaking," so we know he has been on different ships and living a fairly nomadic life for a while now. This has prevented him from making close friends and he is finally feeling the effects. Even in letter two we see that his journey is far from over. He tells us it may still be years before he sees her again!
His loneliness foreshadows some of the loneliness Victor later feels as he, too, isolates himself from his friends and family in pursuit of his own "undertaking" of creating a new species.