It's not dumb as long as you ask questions (and are interested in the answer).

The global ice sheets currently melt at a rate of 1.4 trillion tons per year. Biomass of all living things is about 500 billion tons C. Let's pretend all of that is plant mass on land (most if it isn't), and let's just say there's 5 times as much water as carbon in plants (an overestimation at least for trees), and we arrive at only 2.5 trillion tons of water bound.

So, if I my estimations aren't wildly off, increasing tree biomass even by a large factor won't help us significantly. Add to that that the rate of the ice melting is accelerating, and that the sea levels are also rising due to thermal expansion, and I don't think we can more or less discount the bound water in trees.

In terms of raw volume of water no, sibling comment has done the maths. But there is another interesting effect that's tangentially relevant. Forested areas act as rainwater sponges. When you get heavy rainfall, if you've got nothing but tarmac or even grass, all that water runs straight off and into the nearest river. Which then bursts its banks and floods everything nearby, making building on floodplains a bloody stupid idea.

With forests, though, the depth of the roots means the water can sink more into the ground. It all still ends up in the river eventually, but over a much longer time period. Because the big pulse of water from the rainfall is attenuated, the river has more of a chance to drain away, so you get fewer, and less serious, floods.

The net effect is that while we'll lose land to rising sea levels, in some areas there's the opportunity to reclaim land currently too marginal to occupy. Nowhere near the same land area, but it's a worthwhile effect to be aware of where it applies.

>have this pet theory of mine that planting trees could also mitigate rising sea levels since trees Contain water. Is that dumb ?

Unfortunately, we would likely have to cover more land than there is on the entire surface of the earth to stave off even a meter of sea level rise. For reference, the sea to land-ratio on earth is around 70/30, and a tree of 15 meters at 60 cm diameter is approximately 5 cubic meters in volume, and a tree contains around 30-50% of its volume in water. The average forest density appears to be about 50-100k trees per square kilometer.

We should still plant trees en masse, but as a solution to sea level rise it's probably not viable.

It... kinda is? Trees can't contain that much water and can't compensate for the volume of water coming from glaciers and the like.

That said, trees do help stabilize the soil, mitigating the loss of arable soil in case of heavy rains and flooding or things like landslides.