Water shows anomalous behavior on heating from $0^{0 }\mathrm{to} 4^0$C Its density unexpectedly increases from $0^{0 }\mathrm{to} 4^0$ C and then decreases. Since for the block, density will decrease on heating, up to $4^{0}c$, fraction of block above water will definitely increase and below the water decrease. After that, the decrease in density of water will be more dominant than decrease in density of wood (which is negligible) and hence the portion of block above water will decrease and below the water will decrease.

Hence fraction below the water decrease first then increase.