Though they go mad they shall be sane, Though they sink through the sea they shall rise again; (6-7)
The kind of perseverance we see here is effortless. Those "mad" folks don't have to try to become "sane" after death. They will naturally feel more at peace, just like the folks on the bottom of the sea who will effortlessly "rise again." It's all part of the cycle of life and death.
Though lovers be lost love shall not; And death shall have no dominion. (8-9)
All the other craziness of life seems to disappear in this poem except for love. Love perseveres in its original form, unlike all the "madness" and pain. So we get the feeling that compared to everything else, love is most important and powerful. More powerful, even, than death.
Split all ends up they shan't crack; And death shall have no dominion. (17-18)
Even when our lives split in different directions or feel as if they might "split" us in two, we still persevere. That part of us that endures after death can never "crack." So of course—you probably beat us to it—death shall have no dominion.